Ignore duplicate elements in AddSelection
Refs https://gitlab.com/arkindex/requests/-/issues/173
If an API user sends duplicate element IDs, let's just ignore them (using a set) and happily react to that (instead of a 400).
Refs https://gitlab.com/arkindex/requests/-/issues/173
If an API user sends duplicate element IDs, let's just ignore them (using a set) and happily react to that (instead of a 400).