<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Work on this issue](https://contributors.gitlab.com/manage-issue?action=work&projectId=278964&issueIid=482482) - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=482482) </details> <!--IssueSummary end--> ### 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.