See html-validate form-dup-name for the peer rule concept.

Adds template-no-duplicate-form-names: flags two form controls sharing a name attribute within the same <form> (or template root if no enclosing form). Radio groups and repeated submit-type controls are explicitly allowed to share a name.

Premise

Per HTML spec §4.10.22.4 — Constructing the entry list, the serializer iterates submittable form controls and emits one entry per contributing control. Two regular controls with the same name both contribute, and server-side frameworks that expect a single value will silently see one — usually not the one the author intended.

The spec distinguishes three categories for name-collision purposes:

• Non-submitting — <input type="button"|"reset">, <button type="button"|"reset">. These do not contribute to the form-data entry list at all; their name can't collide with anything.
• Activation-only — <input type="radio">, submit-like controls (<input type="submit">, <button type="submit">). Sharing a name with same-type siblings is the legitimate pattern: a radio group, or multiple submits distinguished by value.
• Regular — everything else. Two regular controls with the same name is a real collision.

Flags

<form>
  <input name='email' />
  <input name='email' />              {{! duplicate }}
</form>

<form>
  <input type='text' name='field' />
  <textarea name='field'></textarea>  {{! different tag, same name }}
</form>

<form>
  <input type='radio' name='a' />
  <input type='submit' name='a' />   {{! both in shared set, but different types }}
</form>

Allows

{{! Radio group — legitimate shared name }}
<form>
  <input type='radio' name='color' value='red' />
  <input type='radio' name='color' value='blue' />
</form>

{{! Multiple submit buttons with same name — legitimate }}
<form>
  <button type='submit' name='action' value='save'>Save</button>
  <button type='submit' name='action' value='publish'>Publish</button>
</form>

{{! Disabled controls don't submit }}
<form>
  <input name='a' />
  <input name='a' disabled />
</form>

{{! Dynamic name — skipped }}
<form>
  <input name={{this.fieldName}} />
  <input name='email' />
</form>

{{! Mutually exclusive branches never render together }}
<form>
  {{#if @edit}}<input name='x' />{{/if}}
  {{#unless @edit}}<input name='x' />{{/unless}}
</form>

Scope

This is a v1 implementation. Not ported from html-validate: name[] array syntax; the <input type="hidden"> + <input type="checkbox"> default-value pattern. Both are rarely seen in Ember apps; can be added on demand.

Opt-in: not added to any preset config.

Prior art