docs: Add Common Pitfall #59 - Layer 2 duplicate Telegram notifications

Bug: Trade #8 (SHORT SOL-PERP, Nov 22, 04:05) sent 13 duplicate notifications
- P&L compounded $11.50 → $155.05 (8.2× actual $18.79)
- Root cause: Layer 2 ghost detection ignored closingInProgress flag
- Rate limit storm: 6,581 failures → Layer 2 triggered 13 times
- Each call sent notification before async DB update completed

Real incident details:
- TP1: +$32.63 (60% closed)
- Runner: -$13.84 (40% closed at breakeven)
- Net: +$18.79 (Drift confirmed)
- Database showed $155.05 (manually corrected)

Fix: Added closingInProgress check before Layer 2 ghost detection
Related: Pitfalls #40 (death spiral), #48 (closingInProgress), #49 (compounding)

.github/copilot-instructions.md

@@ -3477,6 +3477,62 @@ trade.realizedPnL += actualRealizedPnL  // NOT: result.realizedPnL from SDK
     - **Deployment status:** Code committed and pushed, awaiting container rebuild
     - **Lesson:** In financial systems, "it worked this time" is not enough. Implement defense-in-depth protection for ALL critical operations, even when no failure has occurred yet. Better to have protection you never need than need protection you don't have.
 
+59. **Layer 2 ghost detection causing duplicate Telegram notifications (CRITICAL - Fixed Nov 22, 2025):**
+    - **Symptom:** Trade #8 sent 13 duplicate "POSITION CLOSED" notifications with compounding P&L ($11.50 → $155.05)
+    - **Root Cause:** Layer 2 ghost detection (failureCount > 20) didn't check `closingInProgress` flag before calling `handleExternalClosure()`
+    - **Real incident (Nov 22, 04:05 CET):**
+      * SHORT SOL-PERP: TP1 hit (60% closed at +$32.63), runner closed at breakeven (-$13.84)
+      * Actual net P&L: **+$18.79** (confirmed in Drift UI)
+      * Rate limit storm: 6,581 failed close attempts during runner exit
+      * Layer 2 ghost detection triggered every 2 seconds (priceCheckCount > 20)
+      * Called `handleExternalClosure()` 13 times before trade removal completed
+      * Each call sent Telegram notification with compounding P&L
+      * Database final value: $155.05 (8.2× actual, manually corrected to $18.79)
+    - **Why Common Pitfall #48 didn't prevent this:**
+      * `closingInProgress` flag exists for close verification (Pitfall #48)
+      * But Layer 2 ghost detection (death spiral detector) never checked it
+      * Flag only applied when Position Manager initiated the close
+      * Layer 2 runs when close FAILS repeatedly, so flag wasn't set
+    - **Bug sequence:**
+      1. Runner tries to close via `executeExit()` → 429 rate limit error
+      2. Trade stays in monitoring, `priceCheckCount` increments every 2s
+      3. After 20+ failures: Layer 2 checks Drift API, finds position gone
+      4. Calls `handleExternalClosure()` immediately without checking `closingInProgress`
+      5. Async database update still in progress from previous call
+      6. Next monitoring cycle (2s later): priceCheckCount still > 20, Drift still shows gone
+      7. Layer 2 triggers AGAIN → calls `handleExternalClosure()` AGAIN
+      8. Repeats 13 times during rate limit storm (each call sends notification, compounds P&L)
+    - **Fix (lib/trading/position-manager.ts lines 1477-1490):**
+    ```typescript
+    // BEFORE (BROKEN):
+    if (trade.priceCheckCount > 20) {
+      // Check Drift API for ghost position
+      if (!position || Math.abs(position.size) < 0.01) {
+        await this.handleExternalClosure(trade, 'Layer 2: Ghost detected via Drift API')
+        return
+      }
+    }
+    
+    // AFTER (FIXED):
+    if (trade.priceCheckCount > 20 && !trade.closingInProgress) {
+      // Check Drift API for ghost position
+      if (!position || Math.abs(position.size) < 0.01) {
+        // CRITICAL: Mark as closing to prevent duplicate processing
+        trade.closingInProgress = true
+        trade.closeConfirmedAt = Date.now()
+        
+        await this.handleExternalClosure(trade, 'Layer 2: Ghost detected via Drift API')
+        return
+      }
+    }
+    ```
+    - **Impact:** Prevents duplicate notifications and P&L compounding during rate limit storms
+    - **Verification:** Container restarted Nov 22 05:30 CET with fix
+    - **Database correction:** Manually corrected P&L from $155.05 to $18.79 (actual Drift value)
+    - **Related:** Common Pitfall #40 (ghost death spiral), #48 (closingInProgress flag), #49 (P&L compounding)
+    - **Git commit:** b19f156 "critical: Fix Layer 2 ghost detection causing duplicate Telegram notifications"
+    - **Lesson:** ALL code paths that detect external closures must check `closingInProgress` flag, not just the primary close verification path. Rate limit storms can cause monitoring loop to detect same closure dozens of times if flag isn't checked everywhere.
+
 ## File Conventions
 
 - **API routes:** `app/api/[feature]/[action]/route.ts` (Next.js 15 App Router)