One simple solution you can think of for this use a advanced policy violation rule, where actually you can get the future identity vs current identity, compare the role assignments user will be having differently (or you are sure none of the existing assignments will have start and end date same - you can directly check on new identity ) and for the assignments get the StartDate and endDate of the assignments if both are same, throw a policy violation
This should resolve your issue.
Few pages which can help policy violation