can there be two black Bishops after a pawn promotion
It's rare, but some games have been won by promoting a knight because the knight can check a square that a queen can't. Promoting a bishop over a queen is not likely since they check the same square.


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And while I hate to admt it, this forum thread is surely a sign that we miss 21QC;
Agreed.

unrelated, but why would anyone want anything but a queen or a knight (other than to show off)? i guess maybe in rare cases where queen would cause a stalemate
You answered your own question. Since a Q has both R and B moves, you'd only pick a B or R to avoid stalemating your opponent.

I can't see a situation where it would be necessary to promote to a bishop, given that the queen performs the same function.
I have one here self made.

Although....sorry to pick holes....the move prior to the starting position of your composition must be a terrible blunder by black as from any position there is a check available with the queen right?

sometimes under promotion is required otherwise the game will end in stalemate, here is an example https://www.youtube.com/watch?v=TaeeoC-pjk8
yes