diff --git a/multiplayer-todo.md b/multiplayer-todo.md
index a42e5b20..7d985404 100644
--- a/multiplayer-todo.md
+++ b/multiplayer-todo.md
@@ -1191,7 +1191,7 @@ the local/CI/community transport, not a silent production fallback.
 
 | # | Task | Acceptance |
 |---|---|---|
-| 8.14 `[D:8.4,8.5,8.8]` | **IN PROGRESS.** Pure Go queue domain enforces one active ticket per verified player, 10 s heartbeat/30 s expiry, retry-safe create/heartbeat/cancel and deterministic candidate projection; store layer adds a rebuildable candidate-cache boundary | `server/domain/queue.go` and `server/store/candidates.go` cover ownership/expiry/idempotency, deterministic projection, cache loss and atomic rebuild; PostgreSQL row adapter, real Redis index/TTLs and restart/failover integration remain |
+| 8.14 `[D:8.4,8.5,8.8]` | **IN PROGRESS.** Pure Go queue domain enforces one active ticket per verified player under concurrent mutation, 10 s heartbeat/30 s expiry, retry-safe create/heartbeat/cancel and deterministic candidate projection; store layer adds a rebuildable candidate-cache boundary | `server/domain/queue.go` and `server/store/candidates.go` cover ownership/expiry/idempotency, concurrent create fencing, deterministic projection, cache loss and atomic rebuild; PostgreSQL row adapter, real Redis index/TTLs and restart/failover integration remain |
 | 8.15 `[D:7.8,8.3]` | **IN PROGRESS.** Pure Go probe validation treats Steam location as opaque, requires nonce/freshness/region and server-computed RTT, and implements discrepancy quarantine/release | `server/domain/probes.go` and adversarial fixtures cover stale/wrong/forged evidence, the 25 ms/30% threshold, three-sample quarantine and five-clean release; Steam coordinator and regional probe adapters remain |
 | 8.16 `[D:8.14,8.15]` | **IN PROGRESS.** Pure Go candidate/team selection implements the <=100 ms ceiling, pairwise widening tolerance, anchor inclusion, deterministic set/region scoring and balanced team partitioning | `server/domain/matcher.go`, `teams.go` and adversarial fixtures cover no-common-region, tolerance boundaries, lexical ties, mean-rating balance and input-order independence; queue-backed candidate loading and full population fixtures remain |
 | 8.17 `[D:8.14,8.16]` | **IN PROGRESS.** Pure Go proposal policy sends a 10-second response window to every selected human, requires unanimous acceptance, applies exact decline/timeout cooldowns and ranked escalation | `server/domain/proposal.go` and adversarial fixtures cover partial/unanimous response, expiry, replay/conflict and ranked six-player/cooldown rules; casual 6→2 composition, queue precedence and allocation integration remain |
diff --git a/server/domain/queue.go b/server/domain/queue.go
index a2fdf699..81fd272e 100644
--- a/server/domain/queue.go
+++ b/server/domain/queue.go
@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"sort"
 	"strings"
+	"sync"
 	"time"
 )
 
@@ -37,6 +38,7 @@ type queueMutation struct {
 }
 
 type Queue struct {
+	mu        sync.Mutex
 	tickets   map[string]QueueTicket
 	byPlayer  map[string]string
 	mutations map[string]queueMutation
@@ -50,6 +52,8 @@ func NewQueue() *Queue {
 // production adapter must perform the same check in one transaction and use
 // the same idempotency semantics.
 func (q *Queue) Create(playerID, ticketID, idempotencyKey string, candidate Candidate, now time.Time) (QueueTicket, error) {
+	q.mu.Lock()
+	defer q.mu.Unlock()
 	digest := sha256.Sum256([]byte(createPayload(playerID, ticketID, candidate)))
 	if prior, ok := q.mutations[idempotencyKey]; ok {
 		if prior.digest != digest {
@@ -74,6 +78,8 @@ func (q *Queue) Create(playerID, ticketID, idempotencyKey string, candidate Cand
 }
 
 func (q *Queue) Heartbeat(playerID, ticketID, idempotencyKey string, expectedRevision uint64, now time.Time) (QueueTicket, error) {
+	q.mu.Lock()
+	defer q.mu.Unlock()
 	digest := sha256.Sum256([]byte(fmt.Sprintf("heartbeat:%s:%d", ticketID, expectedRevision)))
 	if prior, ok := q.mutations[idempotencyKey]; ok {
 		if prior.digest != digest {
@@ -105,6 +111,8 @@ func (q *Queue) Heartbeat(playerID, ticketID, idempotencyKey string, expectedRev
 }
 
 func (q *Queue) Cancel(playerID, ticketID, idempotencyKey string, expectedRevision uint64, now time.Time) (QueueTicket, error) {
+	q.mu.Lock()
+	defer q.mu.Unlock()
 	digest := sha256.Sum256([]byte(fmt.Sprintf("cancel:%s:%d", ticketID, expectedRevision)))
 	if prior, ok := q.mutations[idempotencyKey]; ok {
 		if prior.digest != digest {
@@ -132,6 +140,12 @@ func (q *Queue) Cancel(playerID, ticketID, idempotencyKey string, expectedRevisi
 }
 
 func (q *Queue) Expire(now time.Time) []QueueTicket {
+	q.mu.Lock()
+	defer q.mu.Unlock()
+	return q.expireLocked(now)
+}
+
+func (q *Queue) expireLocked(now time.Time) []QueueTicket {
 	var expired []QueueTicket
 	for id, ticket := range q.tickets {
 		if (ticket.State == Queued || ticket.State == Proposed) && !now.Before(ticket.ExpiresAt) {
@@ -147,7 +161,9 @@ func (q *Queue) Expire(now time.Time) []QueueTicket {
 }
 
 func (q *Queue) Candidates(now time.Time) []Candidate {
-	q.Expire(now)
+	q.mu.Lock()
+	defer q.mu.Unlock()
+	q.expireLocked(now)
 	result := make([]Candidate, 0)
 	for _, ticket := range q.tickets {
 		if ticket.State == Queued {
diff --git a/server/domain/queue_test.go b/server/domain/queue_test.go
index 4b8e2896..5e5e770f 100644
--- a/server/domain/queue_test.go
+++ b/server/domain/queue_test.go
@@ -3,6 +3,7 @@ package domain
 import (
 	"errors"
 	"reflect"
+	"sync"
 	"testing"
 	"time"
 )
@@ -77,3 +78,32 @@ func TestQueueCreateIdempotencyIncludesCandidatePayload(t *testing.T) {
 		t.Fatalf("changed create payload error = %v", err)
 	}
 }
+
+func TestQueueConcurrentCreateKeepsOneActiveTicketPerPlayer(t *testing.T) {
+	q := NewQueue()
+	now := time.Unix(1000, 0)
+	var wg sync.WaitGroup
+	results := make(chan error, 2)
+	for i := 0; i < 2; i++ {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			id := string(rune('a' + i))
+			_, err := q.Create("same-player", "ticket-"+id, "create-"+id+"-123456", Candidate{TicketID: "ticket-" + id, PlayerID: "same-player", EnqueuedAt: now}, now)
+			results <- err
+		}(i)
+	}
+	wg.Wait()
+	close(results)
+	succeeded := 0
+	for err := range results {
+		if err == nil {
+			succeeded++
+		} else if !errors.Is(err, ErrPlayerQueued) {
+			t.Fatalf("unexpected concurrent create error: %v", err)
+		}
+	}
+	if succeeded != 1 {
+		t.Fatalf("concurrent creates succeeded %d times", succeeded)
+	}
+}