Validate against duplicate or otherwise corrupt content

A lurking bug for sure, but one that would likely cause issues for us elsewhere too. The docs site would probably struggle if duplicates existed. We could benefit from a "validate" mode in this tool that would report on any such issues; we could then define the behavior for invalid content.

_Originally posted by @jeffhandley in https://github.com/carlossanlop/DocsPortingTool/pull/77#discussion_r681089559_