Update user-identity.js to support in case there are duplicated externalIds but different provider

[natthanun]

Description

Related issues

• connect to

Checklist

• [x] New tests added or existing tests modified to cover all changes
• [x] Code conforms with the style guide

[slnode]

Can one of the admins verify this patch? To accept patch and trigger a build add comment ".ok\W+to\W+test."

[dhmlau]

@slnode test please