User Contribution Mapping - Prevent same user from being mapped to multiple placeholders in the UI

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Problem

As noticed during the test session and reported in https://gitlab.com/gitlab-org/manage/import-and-integrate/discussions/-/issues/112#note_2081594210, currently a user who is already assigned to a placeholder can still be selected in the UI to be assigned to a different placeholder.

Proposal

Ideally, users who have already been assigned to a placeholder in can be excluded from the list of available users to be assigned.