Defensive: prevent duplicate agent runs and stacked review items

Two server-side guards so repeated "Run" clicks can't pile up duplicates:
- Dispatcher: returns the in-flight run if one is already Queued/Running for the same
  (seat, task) instead of dispatching another model call.
- Action gate: if an identical action is already Pending for the same (task, agent),
  it keeps that review item rather than stacking another copy in the inbox.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/Modules/TeamUp.Modules.Assembler/Runtime/AgentRunDispatcher.cs

@@ -1,4 +1,5 @@
 using System.Text.Json;
+using Microsoft.EntityFrameworkCore;
 using TeamUp.Modules.Assembler.Domain;
 using TeamUp.Modules.Assembler.Persistence;
 using TeamUp.Modules.Assembler.Queue;
@@ -12,6 +13,18 @@ internal sealed class AgentRunDispatcher(AssemblerDbContext db, JobQueue queue,
 {
     public async Task<Guid> DispatchAsync(Guid seatId, Guid workItemId, CancellationToken cancellationToken = default)
     {
+        // Defensive: never stack a duplicate run while one is already in flight for this (seat, task).
+        // Repeated "Run" clicks return the in-flight run instead of dispatching another model call.
+        var inFlight = await db.AgentRuns
+            .Where(r => r.SeatId == seatId && r.WorkItemId == workItemId
+                && (r.Status == AgentRunStatus.Queued || r.Status == AgentRunStatus.Running))
+            .Select(r => (Guid?)r.Id)
+            .FirstOrDefaultAsync(cancellationToken);
+        if (inFlight is { } existing)
+        {
+            return existing;
+        }
+
         var run = new AgentRun(seatId, workItemId, clock.GetUtcNow());
         db.AgentRuns.Add(run);
         await db.SaveChangesAsync(cancellationToken);

src/Modules/TeamUp.Modules.Governance/Gate/ActionGate.cs

@@ -1,3 +1,4 @@
+using Microsoft.EntityFrameworkCore;
 using TeamUp.Modules.Governance.Domain;
 using TeamUp.Modules.Governance.Persistence;
 using TeamUp.SharedKernel.Access;
@@ -25,6 +26,20 @@ internal sealed class ActionGate(
 
         if (GatePolicy.ShouldHold(proposal.Autonomy, risk))
         {
+            // Defensive: collapse duplicates. If an identical action is already pending for this
+            // (task, agent), keep that one rather than stacking another copy in the review inbox.
+            var existing = await db.ReviewItems
+                .Where(r => r.WorkItemId == proposal.WorkItemId
+                    && r.AgentId == proposal.AgentId
+                    && r.ActionKind == proposal.ActionKind
+                    && r.Status == ReviewStatus.Pending)
+                .Select(r => (Guid?)r.Id)
+                .FirstOrDefaultAsync(cancellationToken);
+            if (existing is { } existingId)
+            {
+                return new GateResult(GateOutcome.Held, existingId);
+            }
+
             var item = new ReviewItem(proposal, clock.GetUtcNow());
             db.ReviewItems.Add(item);
             await db.SaveChangesAsync(cancellationToken);