Add code duplication detection tooling

.golangci.yaml

@@ -73,6 +73,11 @@ linters:
     - whitespace
     - wrapcheck
   settings:
+    dupl:
+      # Token threshold for duplication detection
+      # Lower = more sensitive (catches smaller duplicates)
+      # Default is 150. We use 75 for lint (blocks CI), `mise run dup` uses 50 (advisory)
+      threshold: 75
     errcheck:
       check-type-assertions: true
       check-blank: true

CLAUDE.md

@@ -47,6 +47,27 @@ Integration tests use the `//go:build integration` build tag and are located in
 mise run fmt && mise run lint
 ```
 
+### Code Duplication Prevention
+
+Before implementing Go code, use `/go:discover-related` to find existing utilities and patterns that might be reusable.
+
+**Check for duplication:**
+```bash
+mise run dup           # Comprehensive check (threshold 50) with summary
+mise run dup:staged    # Check only staged files
+mise run lint          # Normal lint includes dupl at threshold 75 (new issues only)
+mise run lint:full     # All issues at threshold 75
+```
+
+**Tiered thresholds:**
+- **75 tokens** (lint/CI) - Blocks on serious duplication (~20+ lines)
+- **50 tokens** (dup) - Advisory, catches smaller patterns (~10+ lines)
+
+When duplication is found:
+1. Check if a helper already exists in `common.go` or nearby utility files
+2. If not, consider extracting the duplicated logic to a shared helper
+3. If duplication is intentional (e.g., test setup), add a `//nolint:dupl` comment with explanation
+
 ## Code Patterns
 
 ### Error Handling

mise.toml

@@ -52,4 +52,56 @@ mkdir completions
 for sh in bash zsh fish; do
 	go run ./cmd/entire/main.go completion "$sh" >"completions/entire.$sh"
 done
-"""
+"""
+
+[tasks.dup]
+description = "Check for code duplication (threshold 50, with summary)"
+run = """
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Create temp files with proper extensions (works on both Linux and macOS)
+tmpdir=$(mktemp -d)
+config="$tmpdir/config.yaml"
+json_out="$tmpdir/output.json"
+
+cat > "$config" << 'YAML'
+version: "2"
+linters:
+  default: none
+  enable: [dupl]
+  settings:
+    dupl:
+      threshold: 50
+YAML
+
+# Run with JSON output for summary, text output for details
+golangci-lint run -c "$config" --new=false --max-issues-per-linter=0 --max-same-issues=0 \
+  --output.json.path="$json_out" --output.text.path=/dev/stderr ./... 2>&1 || true
+
+# Print summary grouped by file
+echo ""
+echo "=== Duplication Summary (by file) ==="
+if command -v jq &>/dev/null && [ -s "$json_out" ]; then
+  jq -r '.Issues // [] | group_by(.Pos.Filename) | map({file: (.[0].Pos.Filename | split("/") | .[-1]), count: length}) | sort_by(-.count) | .[] | "  " + (.count|tostring) + " " + .file' "$json_out" 2>/dev/null || echo "  (no issues)"
+else
+  echo "  (install jq for summary)"
+fi
+
+rm -rf "$tmpdir"
+"""
+
+[tasks."dup:staged"]
+description = "Check duplication in staged files only (threshold 75, same as CI)"
+run = """
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Get staged Go files, preserving paths with spaces using null delimiters throughout
+if ! git diff --cached --name-only -z --diff-filter=ACM | grep -z '\\.go$' | grep -zq .; then
+  echo "No staged Go files to check"
+  exit 0
+fi
+echo "Checking staged files for duplication..."
+git diff --cached --name-only -z --diff-filter=ACM | grep -z '\\.go$' | xargs -0 golangci-lint run --enable-only dupl --new=false --max-issues-per-linter=0 --max-same-issues=0
+"""