Issues with element IDs on duplicate merge page

When merging duplicate contacts, the markup on the page is invalidated by duplicate and incorrect IDs being used to describe certain elements in the table that merges their information.

For 'block' fields the IDs are generated correctly (eg: main_phone_1) - for all others the ID 'main__' is applied, or an incorrect / duplicate ID from a previous element. This is incorrect and invalidates the markup (and isn't very useful anyway).

The issue can be recreated on dmaster by creating a dedupe rule that matches on 'last name', running the rule and trying to merge contacts. Inspect the page HTML to see the problem.

Would be a quick fix in the template to sort this out, either removing it or making it more useful: /templates/CRM/Contact/Form/Merge.tpl

https://github.com/civicrm/civicrm-core/blob/master/templates/CRM/Contact/Form/Merge.tpl#L107-L112