When _reindexObjectList receives a document more than once (which may
happen when multiple different-tag indexation activities are spawned for
the same document), it would emit a meaningless error, saying that
document /foo/bar stole the uid of document /foo/bar.
Instead, fix duplicate tracking and skip such dulicates.

Also, simplify & make _reindexObjectList more robust:
- Always check path length.
- Allocate uids before looking for duplicates in catalog (it may not be
  actually needed at this level nowadays).
- Always check both uid-to-path and path-to-uid mappings.
- Reuse existing mappings to detect duplicates among objects being reindexed,
  removing the need for assigned_uid_dict.
- Avoid computing path more than once, as it's expensive.