If two different users import the same external event into Calendar, we'll currently show both versions if both versions are visible to the viewer.
In most cases, this won't matter (events won't be imported twice, or at most one copy will be visible to the viewer).
However, in some cases it will. Instead of showing all versions of an imported event to a user, we should show only the "best" version, at least when searching via the web UI, where "best" is:
- If the viewer imported a copy themselves, that is the best version.
- Otherwise, the first copy that was imported is the best version.