Don't error out on duplicate id's

It's possible for wikitext to generate invalid html, containing
multiple tags with the same id.  Allow that to pass through.

In general perhaps we need to reconsider throwing exceptions here,
more investigation needs to be done to find out which errors from
libxml are really fatal, and which are just warnings.

Bug: 60785
Change-Id: I827d51e35fa9232259166ef0f9d1f44136def883
Merge "Don't error out on duplicate id's"