[Bug] PostToolUse hook blocking error displayed twice (regression)

Bug Description

PostToolUse hooks that return a blocking error (JSON with "decision": "block") display the error message twice in the terminal and inject two identical <system-reminder> blocks into Claude's context.

This appears to be a regression of previously fixed issues.

Environment Info

• Platform: darwin (macOS)
• Version: 2.1.31
• Terminal: Apple Terminal

Steps to Reproduce

1. Create a minimal test folder:

mkdir -p ~/hook-test/.claude/hooks

1. Create .claude/settings.json:

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Edit|Write",
        "hooks": [
          {
            "type": "command",
            "command": ".claude/hooks/test-hook.sh"
          }
        ]
      }
    ]
  }
}

1. Create .claude/hooks/test-hook.sh:

#!/usr/bin/env bash
echo '{"decision": "block", "reason": "Test error from hook"}'

1. Make it executable: chmod +x .claude/hooks/test-hook.sh

1. Create a test file: echo "test" > test.txt

1. Start Claude Code in that folder and ask it to edit test.txt

Expected behavior

Hook error appears once:

⎿ PostToolUse:Edit hook returned blocking error

Actual behavior

Hook error appears twice:

⎿ PostToolUse:Edit hook returned blocking error
⎿ PostToolUse:Edit hook returned blocking error

Claude also receives two identical <system-reminder> blocks, wasting context tokens.

Related Issues

This appears to be a regression of:

• #9208 - Duplicate PostToolUse:Edit Hook Calls (closed)
• #10871 - Plugin-registered hooks executed twice (closed)
• #14281 - Hook additionalContext injected multiple times (closed, commenter noted "fixed in v2.1")