fix: stop the composer re-sending a queued message already in flight

A queued message is removed from the queue only after its send resolves,
so between dispatch and resolution it stays visible to every reader — and
a composer submit merges the whole queue into its own send. Over a relay
that window is seconds, long enough to deliver the same message twice.

The queue now tracks which entries are awaiting the server. Dispatchers
skip them, clearQueue retains them so the pending send can still remove
or restore its own entry, and the flag is not persisted because a restart
has no in-flight sends.

packages/ui/src/stores/messageQueueStore.ts

@@ -85,6 +85,19 @@ interface MessageQueueState {
     queuedMessages: Record<string, QueuedMessage[]>; // runtime + directory + session → queue
     quarantinedLegacyMessages: Record<string, QueuedMessage[]>;
     followUpBehavior: FollowUpBehavior;
+    /**
+     * Queued messages whose send is currently awaiting the server, per target.
+     *
+     * A queued item is removed only after its send resolves, so between
+     * dispatch and resolution it is still visible to every other reader — and
+     * a composer submit merges the whole queue into its own send. Over a relay
+     * that window is seconds, long enough for the same message to be delivered
+     * twice. Dispatchers must skip entries listed here.
+     *
+     * Never persisted: a restart has no in-flight sends, and a stale flag would
+     * strand a queued message permanently.
+     */
+    sendingIds: Record<string, string[]>;
 }
 
 interface MessageQueueActions {
@@ -94,6 +107,9 @@ interface MessageQueueActions {
     popToInput: (target: MessageQueueTarget, messageId: string) => QueuedMessage | null;
     clearQueue: (target: MessageQueueTarget) => void;
     clearAllQueues: () => void;
+    markSending: (target: MessageQueueTarget, messageId: string) => void;
+    clearSending: (target: MessageQueueTarget, messageId: string) => void;
+    getSendableQueue: (target: MessageQueueTarget) => QueuedMessage[];
     setFollowUpBehavior: (behavior: FollowUpBehavior) => void;
     getQueueForTarget: (target: MessageQueueTarget) => QueuedMessage[];
 }
@@ -127,6 +143,7 @@ export const useMessageQueueStore = create<MessageQueueStore>()(
                 queuedMessages: {},
                 quarantinedLegacyMessages: {},
                 followUpBehavior: DEFAULT_FOLLOW_UP_BEHAVIOR,
+                sendingIds: {},
 
                 addToQueue: (target, message) => {
                     const key = getMessageQueueKey(target);
@@ -237,6 +254,14 @@ export const useMessageQueueStore = create<MessageQueueStore>()(
                 clearQueue: (target) => {
                     const key = getMessageQueueKey(target);
                     set((state) => {
+                        // Clearing drops what is still queued, never a message
+                        // already handed to the server: that send will resolve
+                        // and must find its entry to remove or restore.
+                        const sending = state.sendingIds[key] ?? [];
+                        const retained = (state.queuedMessages[key] ?? []).filter((m) => sending.includes(m.id));
+                        if (retained.length > 0) {
+                            return { queuedMessages: { ...state.queuedMessages, [key]: retained } };
+                        }
                         const { [key]: _removed, ...rest } = state.queuedMessages;
                         void _removed;
                         return { queuedMessages: rest };
@@ -244,7 +269,40 @@ export const useMessageQueueStore = create<MessageQueueStore>()(
                 },
 
                 clearAllQueues: () => {
-                    set({ queuedMessages: {} });
+                    set({ queuedMessages: {}, sendingIds: {} });
+                },
+
+                markSending: (target, messageId) => {
+                    const key = getMessageQueueKey(target);
+                    set((state) => {
+                        const current = state.sendingIds[key] ?? [];
+                        if (current.includes(messageId)) return state;
+                        return { sendingIds: { ...state.sendingIds, [key]: [...current, messageId] } };
+                    });
+                },
+
+                clearSending: (target, messageId) => {
+                    const key = getMessageQueueKey(target);
+                    set((state) => {
+                        const current = state.sendingIds[key];
+                        if (!current || !current.includes(messageId)) return state;
+                        const next = current.filter((id) => id !== messageId);
+                        if (next.length === 0) {
+                            const { [key]: _removed, ...rest } = state.sendingIds;
+                            void _removed;
+                            return { sendingIds: rest };
+                        }
+                        return { sendingIds: { ...state.sendingIds, [key]: next } };
+                    });
+                },
+
+                getSendableQueue: (target) => {
+                    const key = getMessageQueueKey(target);
+                    const state = get();
+                    const queue = state.queuedMessages[key] ?? [];
+                    const sending = state.sendingIds[key];
+                    if (!sending || sending.length === 0) return queue;
+                    return queue.filter((message) => !sending.includes(message.id));
                 },
 
                 setFollowUpBehavior: (behavior) => {