If you have two queens (original and one promoted) on board and the original queen is captured....can you promote your next pawn at the eight rank to queen again?
Yes, you can.
Fide rules of chess 3.7.e):
When a player, having the move, plays a pawn to the rank furthest from its starting position, he must exchange that pawn as part of the same move for a new queen, rook, bishop or knight of the same colour on the intended square of arrival. This is called the square of ‘promotion’. The player's choice is not restricted to pieces that have been captured previously. This exchange of a pawn for another piece is called promotion, and the effect of the new piece is immediate.
(bold by me)