A position ID identifies a staffing slot. More than one active worker assigned to that ID can indicate a duplicate assignment under this tenant's one-worker-per-position rule. Two workers with the same job title aren't enough evidence; job titles describe roles, and legitimate positions often share them.
Group active worker records by position ID and count worker instances. Then inspect the contributing IDs for groups above one. Compare that worker-side evidence with the position record's worker reference instead of assuming the position table contains every conflicting assignment. The vacancy register answers another question because an open position has no current worker. Preserve terminated workers in historical analysis, but exclude their former assignments when checking this current-state rule.