diff --git a/server/consumer.go b/server/consumer.go
index e4118c72cf5..bb248f7c430 100644
--- a/server/consumer.go
+++ b/server/consumer.go
@@ -2271,10 +2271,27 @@ func configsEqualSansDelivery(a, b ConsumerConfig) bool {
 }
 
 // Helper to send a reply to an ack.
-func (o *consumer) sendAckReply(subj string) {
+func (o *consumer) sendAckReply(subj string, err *ApiError) {
 	o.mu.RLock()
 	defer o.mu.RUnlock()
-	o.outq.sendMsg(subj, nil)
+	o.sendAckReplyLocked(subj, err)
+}
+
+// Helper to send a reply to an ack.
+// Lock must be held
+func (o *consumer) sendAckReplyLocked(subj string, err *ApiError) {
+
+	var resp = JSApiConsumerAckResponse{ApiResponse: ApiResponse{Type: JSApiConsumerAckResponseType, Error: err}}
+
+	if err == nil {
+		resp.Success = true
+	}
+
+	j, e := json.Marshal(resp)
+	if e != nil {
+		return
+	}
+	o.outq.sendMsg(subj, j)
 }
 
 type jsAckMsg struct {
@@ -2327,17 +2344,20 @@ func (o *consumer) processAck(subject, reply string, hdr int, rmsg []byte) {
 	}
 
 	sseq, dseq, dc := ackReplyInfo(subject)
+	var err *ApiError
 
 	skipAckReply := sseq == 0
 
 	switch {
 	case len(msg) == 0, bytes.Equal(msg, AckAck), bytes.Equal(msg, AckOK):
-		if !o.processAckMsg(sseq, dseq, dc, reply, true) {
+		if b, e := o.processAckMsg(sseq, dseq, dc, reply, true); e == nil && !b {
 			// We handle replies for acks in updateAcks
 			skipAckReply = true
+		} else {
+			err = e
 		}
 	case bytes.HasPrefix(msg, AckNext):
-		o.processAckMsg(sseq, dseq, dc, _EMPTY_, true)
+		_, _ = o.processAckMsg(sseq, dseq, dc, _EMPTY_, true)
 		o.processNextMsgRequest(reply, msg[len(AckNext):])
 		skipAckReply = true
 	case bytes.HasPrefix(msg, AckNak):
@@ -2357,7 +2377,7 @@ func (o *consumer) processAck(subject, reply string, hdr int, rmsg []byte) {
 
 	// Ack the ack if requested.
 	if len(reply) > 0 && !skipAckReply {
-		o.sendAckReply(reply)
+		o.sendAckReply(reply, err)
 	}
 }
 
@@ -2493,7 +2513,7 @@ func (o *consumer) addAckReply(sseq uint64, reply string) {
 }
 
 // Lock should be held.
-func (o *consumer) updateAcks(dseq, sseq uint64, reply string) {
+func (o *consumer) updateAcks(dseq, sseq uint64, reply string, err *ApiError) {
 	if o.node != nil {
 		// Inline for now, use variable compression.
 		var b [2*binary.MaxVarintLen64 + 1]byte
@@ -2509,7 +2529,7 @@ func (o *consumer) updateAcks(dseq, sseq uint64, reply string) {
 		o.store.UpdateAcks(dseq, sseq)
 		if reply != _EMPTY_ {
 			// Already locked so send direct.
-			o.outq.sendMsg(reply, nil)
+			o.sendAckReplyLocked(reply, err)
 		}
 	}
 	// Update activity.
@@ -2705,7 +2725,7 @@ func (o *consumer) processNak(sseq, dseq, dc uint64, nak []byte) {
 // case the reply will be sent later).
 func (o *consumer) processTerm(sseq, dseq, dc uint64, reason, reply string) bool {
 	// Treat like an ack to suppress redelivery.
-	ackedInPlace := o.processAckMsg(sseq, dseq, dc, reply, false)
+	ackedInPlace, _ := o.processAckMsg(sseq, dseq, dc, reply, false)
 
 	o.mu.Lock()
 	defer o.mu.Unlock()
@@ -3061,17 +3081,17 @@ func (o *consumer) sampleAck(sseq, dseq, dc uint64) {
 // Returns `true` if the ack was processed in place and the sender can now respond
 // to the client, or `false` if there was an error or the ack is replicated (in which
 // case the reply will be sent later).
-func (o *consumer) processAckMsg(sseq, dseq, dc uint64, reply string, doSample bool) bool {
+func (o *consumer) processAckMsg(sseq, dseq, dc uint64, reply string, doSample bool) (bool, *ApiError) {
 	o.mu.Lock()
 	if o.closed {
 		o.mu.Unlock()
-		return false
+		return false, nil
 	}
 
 	mset := o.mset
 	if mset == nil || mset.closed.Load() {
 		o.mu.Unlock()
-		return false
+		return false, nil
 	}
 
 	// Check if this ack is above the current pointer to our next to deliver.
@@ -3083,11 +3103,8 @@ func (o *consumer) processAckMsg(sseq, dseq, dc uint64, reply string, doSample b
 		var ss StreamState
 		mset.store.FastState(&ss)
 		if sseq > ss.LastSeq {
-			o.srv.Warnf("JetStream consumer '%s > %s > %s' ACK sequence %d past last stream sequence of %d",
-				o.acc.Name, o.stream, o.name, sseq, ss.LastSeq)
-			// FIXME(dlc) - For 2.11 onwards should we return an error here to the caller?
 			o.mu.Unlock()
-			return false
+			return false, NewJSConsumerMsgNotPendingAckError()
 		}
 		o.sseq = sseq + 1
 	}
@@ -3099,6 +3116,7 @@ func (o *consumer) processAckMsg(sseq, dseq, dc uint64, reply string, doSample b
 
 	var sgap, floor uint64
 	var needSignal bool
+	var err *ApiError
 
 	switch o.cfg.AckPolicy {
 	case AckExplicit:
@@ -3128,14 +3146,17 @@ func (o *consumer) processAckMsg(sseq, dseq, dc uint64, reply string, doSample b
 					}
 				}
 			}
+		} else {
+			err = NewJSConsumerMsgNotPendingAckError()
 		}
+
 		delete(o.rdc, sseq)
 		o.removeFromRedeliverQueue(sseq)
 	case AckAll:
 		// no-op
 		if dseq <= o.adflr || sseq <= o.asflr {
 			o.mu.Unlock()
-			return ackInPlace
+			return ackInPlace, nil
 		}
 		if o.maxp > 0 && len(o.pending) >= o.maxp {
 			needSignal = true
@@ -3167,7 +3188,7 @@ func (o *consumer) processAckMsg(sseq, dseq, dc uint64, reply string, doSample b
 	case AckNone:
 		// FIXME(dlc) - This is error but do we care?
 		o.mu.Unlock()
-		return ackInPlace
+		return ackInPlace, nil
 	}
 
 	// No ack replication, so we set reply to "" so that updateAcks does not
@@ -3176,7 +3197,7 @@ func (o *consumer) processAckMsg(sseq, dseq, dc uint64, reply string, doSample b
 		reply = _EMPTY_
 	}
 	// Update underlying store.
-	o.updateAcks(dseq, sseq, reply)
+	o.updateAcks(dseq, sseq, reply, err)
 	o.mu.Unlock()
 
 	if ackInPlace {
@@ -3194,7 +3215,7 @@ func (o *consumer) processAckMsg(sseq, dseq, dc uint64, reply string, doSample b
 	if needSignal {
 		o.signalNewMessages()
 	}
-	return ackInPlace
+	return ackInPlace, err
 }
 
 // Determine if this is a truly filtered consumer. Modern clients will place filtered subjects
@@ -4834,7 +4855,7 @@ func (o *consumer) deliverMsg(dsubj, ackReply string, pmsg *jsPubMsg, dc uint64,
 		if mset != nil && mset.ackq != nil && (o.node == nil || o.cfg.Direct) {
 			mset.ackq.push(seq)
 		} else {
-			o.updateAcks(dseq, seq, _EMPTY_)
+			o.updateAcks(dseq, seq, _EMPTY_, nil)
 		}
 	}
 }
diff --git a/server/errors.json b/server/errors.json
index 9bb8cb18c66..92ab970d4e2 100644
--- a/server/errors.json
+++ b/server/errors.json
@@ -1628,5 +1628,15 @@
     "help": "",
     "url": "",
     "deprecates": ""
+  },
+  {
+    "constant": "JSConsumerMsgNotPendingAckErr",
+    "code": 400,
+    "error_code": 10165,
+    "description": "This message is currently not pending an ack (already acked or not yet delivered)",
+    "comment": "",
+    "help": "",
+    "url": "",
+    "deprecates": ""
   }
 ]
diff --git a/server/jetstream_api.go b/server/jetstream_api.go
index 45a6ddc01df..c3011ab7037 100644
--- a/server/jetstream_api.go
+++ b/server/jetstream_api.go
@@ -718,6 +718,13 @@ type JSApiConsumerPauseRequest struct {
 	PauseUntil time.Time `json:"pause_until,omitempty"`
 }
 
+const JSApiConsumerAckResponseType = "io.nats.jetstream.api.v1.consumer_ack_response"
+
+type JSApiConsumerAckResponse struct {
+	ApiResponse
+	Success bool `json:"success,omitempty"`
+}
+
 const JSApiConsumerPauseResponseType = "io.nats.jetstream.api.v1.consumer_pause_response"
 
 type JSApiConsumerPauseResponse struct {
diff --git a/server/jetstream_cluster.go b/server/jetstream_cluster.go
index 93f681c323d..e57d2244ac6 100644
--- a/server/jetstream_cluster.go
+++ b/server/jetstream_cluster.go
@@ -5275,7 +5275,7 @@ func (o *consumer) processReplicatedAck(dseq, sseq uint64) error {
 
 	// Check if we have a reply that was requested.
 	if reply := o.replies[sseq]; reply != _EMPTY_ {
-		o.outq.sendMsg(reply, nil)
+		o.sendAckReplyLocked(reply, nil)
 		delete(o.replies, sseq)
 	}
 
diff --git a/server/jetstream_cluster_1_test.go b/server/jetstream_cluster_1_test.go
index 06adf875c5d..30c4fabc00e 100644
--- a/server/jetstream_cluster_1_test.go
+++ b/server/jetstream_cluster_1_test.go
@@ -6668,9 +6668,11 @@ func TestJetStreamClusterMetaRecoveryConsumerCreateAndRemove(t *testing.T) {
 	}
 }
 
-// Make sure if we received acks that are out of bounds, meaning past our
+// Test that acking returns the expected reply message.
+// Make sure that an error is returned when trying to ack the same message more than once.
+// Make sure if we receive acks that are out of bounds, meaning past our
 // last sequence or before our first that they are ignored and errored if applicable.
-func TestJetStreamClusterConsumerAckOutOfBounds(t *testing.T) {
+func TestJetStreamConsumerClusterAckAck(t *testing.T) {
 	c := createJetStreamClusterExplicit(t, "R3S", 3)
 	defer c.shutdown()
 
@@ -6696,13 +6698,33 @@ func TestJetStreamClusterConsumerAckOutOfBounds(t *testing.T) {
 	msgs, err := sub.Fetch(1)
 	require_NoError(t, err)
 	require_Equal(t, len(msgs), 1)
-	msgs[0].AckSync()
+	ackSubject := msgs[0].Reply
+
+	var resp JSApiConsumerAckResponse
+
+	// first ack
+	replyMsg, err := nc.Request(ackSubject, nil, 5000*time.Millisecond)
+	require_NoError(t, err)
+	require_NoError(t, json.Unmarshal(replyMsg.Data, &resp))
+	require_True(t, resp.Success)
+
+	// check for error if trying to ack again
+	resp = JSApiConsumerAckResponse{}
+	replyMsg, err = nc.Request(ackSubject, nil, 250*time.Millisecond)
+	require_NoError(t, err)
+	require_NoError(t, json.Unmarshal(replyMsg.Data, &resp))
+	require_False(t, resp.Success)
+	require_Equal(t, resp.Error.ErrCode, uint16(JSConsumerMsgNotPendingAckErr))
 
 	// Now ack way past the last sequence.
-	_, err = nc.Request("$JS.ACK.TEST.C.1.10000000000.0.0.0", nil, 250*time.Millisecond)
-	require_Error(t, err, nats.ErrTimeout)
+	resp = JSApiConsumerAckResponse{}
+	replyMsg, err = nc.Request("$JS.ACK.TEST.C.1.10000000000.0.0.0", nil, 250*time.Millisecond)
+	require_NoError(t, err)
+	require_NoError(t, json.Unmarshal(replyMsg.Data, &resp))
+	require_False(t, resp.Success)
+	require_Equal(t, resp.Error.ErrCode, uint16(JSConsumerMsgNotPendingAckErr))
 
-	// Make sure that now changes happened to our state.
+	// Make sure that no changes happened to our state.
 	ci, err := js.ConsumerInfo("TEST", "C")
 	require_NoError(t, err)
 	require_Equal(t, ci.Delivered.Consumer, 1)
diff --git a/server/jetstream_errors_generated.go b/server/jetstream_errors_generated.go
index 146e969eb7c..6f148f60b7a 100644
--- a/server/jetstream_errors_generated.go
+++ b/server/jetstream_errors_generated.go
@@ -158,6 +158,9 @@ const (
 	// JSConsumerMetadataLengthErrF consumer metadata exceeds maximum size of {limit}
 	JSConsumerMetadataLengthErrF ErrorIdentifier = 10135
 
+	// JSConsumerMsgNotPendingAckErr This message is currently not pending an ack (already acked or not yet delivered)
+	JSConsumerMsgNotPendingAckErr ErrorIdentifier = 10165
+
 	// JSConsumerMultipleFiltersNotAllowed consumer with multiple subject filters cannot use subject based API
 	JSConsumerMultipleFiltersNotAllowed ErrorIdentifier = 10137
 
@@ -548,6 +551,7 @@ var (
 		JSConsumerMaxRequestExpiresToSmall:         {Code: 400, ErrCode: 10115, Description: "consumer max request expires needs to be >= 1ms"},
 		JSConsumerMaxWaitingNegativeErr:            {Code: 400, ErrCode: 10087, Description: "consumer max waiting needs to be positive"},
 		JSConsumerMetadataLengthErrF:               {Code: 400, ErrCode: 10135, Description: "consumer metadata exceeds maximum size of {limit}"},
+		JSConsumerMsgNotPendingAckErr:              {Code: 400, ErrCode: 10165, Description: "This message is currently not pending an ack (already acked or not yet delivered)"},
 		JSConsumerMultipleFiltersNotAllowed:        {Code: 400, ErrCode: 10137, Description: "consumer with multiple subject filters cannot use subject based API"},
 		JSConsumerNameContainsPathSeparatorsErr:    {Code: 400, ErrCode: 10127, Description: "Consumer name can not contain path separators"},
 		JSConsumerNameExistErr:                     {Code: 400, ErrCode: 10013, Description: "consumer name already in use"},
@@ -1249,6 +1253,16 @@ func NewJSConsumerMetadataLengthError(limit interface{}, opts ...ErrorOption) *A
 	}
 }
 
+// NewJSConsumerMsgNotPendingAckError creates a new JSConsumerMsgNotPendingAckErr error: "This message is currently not pending an ack (already acked or not yet delivered)"
+func NewJSConsumerMsgNotPendingAckError(opts ...ErrorOption) *ApiError {
+	eopts := parseOpts(opts)
+	if ae, ok := eopts.err.(*ApiError); ok {
+		return ae
+	}
+
+	return ApiErrors[JSConsumerMsgNotPendingAckErr]
+}
+
 // NewJSConsumerMultipleFiltersNotAllowedError creates a new JSConsumerMultipleFiltersNotAllowed error: "consumer with multiple subject filters cannot use subject based API"
 func NewJSConsumerMultipleFiltersNotAllowedError(opts ...ErrorOption) *ApiError {
 	eopts := parseOpts(opts)
diff --git a/server/jetstream_test.go b/server/jetstream_test.go
index 66b7566fb05..3a1649cdd39 100644
--- a/server/jetstream_test.go
+++ b/server/jetstream_test.go
@@ -2995,8 +2995,8 @@ func TestJetStreamConsumerAckAck(t *testing.T) {
 	nc := clientConnectToServer(t, s)
 	defer nc.Close()
 
-	// 4 for number of ack protocols to test them all.
-	for i := 0; i < 4; i++ {
+	// 5 for number of ack protocols plus already acked to test them all.
+	for i := 0; i < 5; i++ {
 		sendStreamMsg(t, nc, mname, "Hello World!")
 	}
 
@@ -3015,6 +3015,34 @@ func TestJetStreamConsumerAckAck(t *testing.T) {
 	testAck(AckNak)
 	testAck(AckProgress)
 	testAck(AckTerm)
+
+	// checking replies and errors on two explicit acks for the same message or out of range ack
+	// first get the message
+	m, err := nc.Request(rqn, nil, 10*time.Millisecond)
+	require_NoError(t, err)
+
+	var resp JSApiConsumerAckResponse
+	// Send a request for the first ack and make sure it worked.
+	ackReply1, err := nc.Request(m.Reply, AckAck, 10*time.Millisecond)
+	require_NoError(t, err)
+	require_NoError(t, json.Unmarshal(ackReply1.Data, &resp))
+	require_True(t, resp.Success)
+
+	// Send a second ack and make sure it fails.
+	ackReply2, err := nc.Request(m.Reply, AckAck, 10*time.Millisecond)
+	require_NoError(t, err)
+	resp = JSApiConsumerAckResponse{}
+	require_NoError(t, json.Unmarshal(ackReply2.Data, &resp))
+	require_False(t, resp.Success)
+	require_Equal(t, resp.Error.ErrCode, uint16(JSConsumerMsgNotPendingAckErr))
+
+	// Error trying to ack an out of range sequence number
+	ackReply3, err := nc.Request("$JS.ACK.ACK-ACK.worker.1.6.0.0.0", AckAck, 10*time.Millisecond)
+	require_NoError(t, err)
+	resp = JSApiConsumerAckResponse{}
+	require_NoError(t, json.Unmarshal(ackReply3.Data, &resp))
+	require_False(t, resp.Success)
+	require_Equal(t, resp.Error.ErrCode, uint16(JSConsumerMsgNotPendingAckErr))
 }
 
 func TestJetStreamAckNext(t *testing.T) {