# Duplicate Detection on Create

## Objective

Warn users before they create something that probably already exists.

A check on create that surfaces likely existing matches and lets the user reuse one instead of adding another copy.

## Before You Begin

This feature is being added to an application that already exists and already
works. Do not scaffold a new project, and do not assume a blank slate.

Inspect the codebase first and establish:

- The existing application structure and where code of this kind already lives.
- The framework and version in use.
- The existing design system — colours, spacing, typography, and component conventions.
- Existing UI components you can reuse instead of writing new ones.
- The existing database structure, if this feature needs to persist anything.
- The existing authentication and authorization system, if this feature is user-scoped.
- Dependencies already installed, so you don't add a library that duplicates one.
- The existing test setup and conventions.

Only start writing code once you understand the above. If the application
already implements part of this feature, extend it rather than replacing it.

## Implementation Instructions

1. Pick the fields that actually identify a record — name, email, phone, domain — and match on those rather than on everything.
2. Normalize before comparing: trim and collapse whitespace, fold case, strip punctuation, canonicalize phone numbers, and handle email plus-addressing and dots consistently.
3. Show possible matches inline before the user commits, with enough detail to tell them apart, and offer to open the existing record instead.
4. Recheck on the server at write time. A client-side check loses the race when two people submit at once.
5. Do NOT hard-block on a fuzzy match. Similar names are often different people. Let the user override, and record who overrode and why.

## UI and UX Requirements

Match the application's existing design system exactly. Reuse its components,
spacing, and typography. This feature should look like it was always there.

## Responsive Requirements

Works on mobile, tablet, and desktop. Touch targets are large enough to hit on a
phone, and nothing overflows horizontally at 320px.

## Accessibility Requirements

- Fully keyboard navigable.
- Correct semantic elements and ARIA roles.
- Visible focus states.
- Meets WCAG AA contrast.
- Dynamic changes are announced to screen readers.
- Respects prefers-reduced-motion.

## Edge Cases

- Matching must respect tenant and permission boundaries — never reveal a record the user cannot otherwise see.
- A near-match the user cannot view must be silently ignored, not shown as a redacted teaser.
- Editing an existing record must not flag that record as a duplicate of itself.
- The scan must stay fast on large tables; index the normalized comparison values rather than scanning raw text.
- Genuinely identical names are common in some datasets. If duplicates are legitimate for an object type, do not check it at all.
- An override must be auditable, so someone can later ask why two records exist.

## Testing

Exercise the feature end to end in the running application. Cover every edge case
above, then run the existing test suite and confirm nothing regressed.

## Acceptance Criteria

- [ ] Likely duplicates are shown before the record is created.
- [ ] Comparison values are normalized consistently on read and write.
- [ ] The server rechecks at write time and prevents concurrent duplicates.
- [ ] Matches are scoped to what the user is permitted to see.
- [ ] Users can override the warning and the override is recorded.
- [ ] Editing a record never flags it against itself.
- [ ] The feature matches the existing design system.
- [ ] No existing functionality is broken.

## Adaptation Rules

- Match the existing design system. Do not introduce a new colour palette,
  spacing scale, or component library.
- Reuse existing components and utilities wherever they fit.
- Follow the naming, file layout, and code style already present.
- Do not upgrade, replace, or remove existing dependencies to make this
  feature fit. Adapt the feature to the app, not the app to the feature.
- Do not break existing functionality. If a change is genuinely required in
  existing code, make the smallest one that works and say so.
- If something in these instructions conflicts with how the application is
  built, follow the application and explain the deviation.

## Final Verification

Before you report the work as done:

1. Re-read the acceptance criteria above and check each one against what you
   actually built.
2. Run the application and exercise the feature end to end.
3. Run the existing test suite and confirm you have broken nothing.
4. Check the feature on mobile, tablet, and desktop widths.
5. Check keyboard navigation and focus handling.
6. Summarize what changed: files added, files modified, and anything you
   deliberately did differently because of how this application is built.

If any acceptance criterion is unmet, fix it before reporting completion.