I understand that you can pick up the top discard (and then the whole discard pile) if the pile is not frozen, and the top card matches a meld you have on the table.
I have some friends who play that you cannot do this if the meld is already a canasta. That makes a matching card a much safer discard - the opponent could conceivably still take the pile with matching cards from their hand, but if they already have a canasta of the same cards on the table, this is much less likely.
Any opinions, or definitive rules on this?