In Canasta, if the discard pile has already been frozen with a wild card, is a subsequent discard allowed to be another wild card?
You can always discard a wild card even if one is already on the pile.  It freezes the pile for both sides, much like the start of the hand.
When the pile is frozen, it can only be unfrozen when a natural (non wild or 3) is on top of the pile and it is taken with a natural pair of those same cards.
I've played several different variants of canasta, but the above rules are the same.
Here is one set of rules for Canasta, I'm not aware of an official set of rules from a universal governing body.  https://bicyclecards.com/how-to-play/canasta/