docs: Add bug report for duplicate tool_result blocks issue

Documents API error where multiple tool_result blocks are sent with
the same tool_use_id when a tool fails. The "Sibling tool call errored"
message is incorrectly being sent for the same tool that failed,
rather than only for other sibling tools in a parallel batch.

Includes:
- Root cause analysis
- Example JSON from reported incidents
- Suggested fix locations and pseudocode
- Related request IDs for debugging

.github/issues/duplicate-tool-result-bug.md

@@ -0,0 +1,123 @@
+# Bug Report: Duplicate tool_result blocks with same tool_use_id
+
+## Summary
+When a tool call fails (particularly observed with the Tmux tool), two `tool_result` blocks are incorrectly sent to the API with the same `tool_use_id`, causing an API error:
+
+```
+API Error: 400
+{"type":"error","error":{"type":"invalid_request_error","message":"messages.X.content.1: each tool_use must have a single result. Found multiple `tool_result` blocks with id: toolu_XXX"}}
+```
+
+## Root Cause
+The parallel tool execution error handling code sends a "Sibling tool call errored" message even for the tool that actually failed, when it should only send this message for OTHER sibling tools in the parallel batch.
+
+## Reproduction
+1. Execute a tool call that will fail (e.g., Tmux select-pane on a non-existent window)
+2. Observe two tool_result messages being generated:
+   - First: The actual error result ("Window not found...")
+   - Second: A "<tool_use_error>Sibling tool call errored</tool_use_error>" message with the SAME tool_use_id
+
+## Example JSON (from reported incident)
+```json
+// Assistant requests tool use
+{
+  "type": "tool_use",
+  "id": "toolu_01K2K2KFvUrwA9PaHGp652zW",
+  "name": "Tmux",
+  "input": {"args": ["select-pane", "-t", "test-perms3:0.0"]}
+}
+
+// First tool_result (correct - actual error)
+{
+  "type": "user",
+  "message": {
+    "role": "user",
+    "content": [{
+      "tool_use_id": "toolu_01K2K2KFvUrwA9PaHGp652zW",
+      "type": "tool_result",
+      "content": "Window not found. The specified window may not exist.",
+      "is_error": true
+    }]
+  }
+}
+
+// Second tool_result (BUG - same tool_use_id!)
+{
+  "type": "user",
+  "message": {
+    "role": "user",
+    "content": [{
+      "type": "tool_result",
+      "content": "<tool_use_error>Sibling tool call errored</tool_use_error>",
+      "is_error": true,
+      "tool_use_id": "toolu_01K2K2KFvUrwA9PaHGp652zW"
+    }]
+  }
+}
+```
+
+## Expected Behavior
+- When a tool fails, only ONE tool_result should be sent for that tool_use_id
+- The "Sibling tool call errored" message should ONLY be sent for OTHER tool_use_ids in a parallel batch, not for the tool that actually errored
+
+## Suggested Fix Location
+The fix should be in the code that handles parallel tool execution errors:
+
+```typescript
+// Pseudocode for the fix
+function handleParallelToolResults(toolUseIds: string[], failedToolId: string, error: Error) {
+  const results = [];
+
+  for (const id of toolUseIds) {
+    if (id === failedToolId) {
+      // Send actual error for the failed tool
+      results.push({
+        tool_use_id: id,
+        type: "tool_result",
+        content: error.message,
+        is_error: true
+      });
+    } else {
+      // Only send sibling error for OTHER tools
+      results.push({
+        tool_use_id: id,
+        type: "tool_result",
+        content: "<tool_use_error>Sibling tool call errored</tool_use_error>",
+        is_error: true
+      });
+    }
+  }
+
+  return results;
+}
+```
+
+Additionally, a deduplication check before sending to the API would prevent this:
+```typescript
+function deduplicateToolResults(messages: Message[]): Message[] {
+  const seenToolUseIds = new Set<string>();
+  return messages.filter(msg => {
+    if (msg.type === "tool_result") {
+      if (seenToolUseIds.has(msg.tool_use_id)) {
+        return false; // Skip duplicate
+      }
+      seenToolUseIds.add(msg.tool_use_id);
+    }
+    return true;
+  });
+}
+```
+
+## Related CHANGELOG Fixes
+Similar issues have been fixed before:
+- v1.0.84: "Fix tool_use/tool_result id mismatch error when network is unstable"
+- v2.0.0: "Hooks: Reduced PostToolUse 'tool_use' ids were found without 'tool_result' blocks errors"
+- v2.0.22: "Fix bug causing duplicate permission prompts with parallel tool calls"
+
+## Reported By
+Issue reported via Slack with multiple reproductions showing the Tmux tool triggering this bug.
+
+## Request IDs (for debugging)
+- req_011CVt2KXhCGmrL2YN9sSZQP
+- req_011CVtFYJLPxn9oB9FEkCnRr
+- req_011CVtG1PREveTLikakXGEdn