Validate `add` to prevent duplicate IDs.

[levand]

Description of changes

Throw errors if the user attempts to add duplicate IDs, either via separate calls to Collection.add or the same one.

Test plan

This branch is based of #355 which contains failing tests for this functionality.

Documentation Changes

https://github.com/chroma-core/docs/pull/42

[atroyn]

@levand let's find time to review in person

[levand]

Reviewed in-person with @atroyn and incorporated feedback prior to merging.