Forms should throw error if two or more NgModels appear in form with the same name

[kemsky]

[ ] bug report => search github for a similar issue or PR before submitting
[x] feature request
[ ] support request => Please do not submit support request here, instead see https://github.com/angular/angular/blob/master/CONTRIBUTING.md#question

Current behavior

Currently there is no error or warning if two form controls have the same name:

<input name="entireStateCoverage" type="checkbox" [(ngModel)]="item.entireStateCoverage">
<input name="entireStateCoverage" type="checkbox" [(ngModel)]="item.active">

Expected behavior

Forms should throw error if duplicate names are detected.

What is the motivation / use case for changing the behavior?

This leads to unpredictable results (value overwrites) which are difficult to find.

• Angular version: 2.4.3

• Browser: [all]

• Language: [all]

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[angular-robot]

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[dylhunn]

We'd likely accept a contribution in this area, and it's good for a first-time contributor. The message would go in forms/src/directives/template_driven_errors.ts, and the check would be somewhere in forms/src/directives.

[phalgunv]

I'd like to work on this issue.