Yes--I do understand, and your code works perfectly well for that initial case. Then the OP shows the following case:
Had the OP used $string2="FFFFF";, your code would not show "F", even though--after a string comparison--"F" is unique to $string2.
I see many good minds working on this issue with different POVs. Again, I think it evidences that the specs are equivocal.
BTW - I didn't downvote yours or others' similar interpretation of "unique;" I merely commented on it.