2 players is in pot.After turn one of goes for all in and other player called his all in.Dealer open the river card without asking players "how many times would you like to open last card?".So there is a dealer mistake here.Other players wins a pot with lucky flush draw with the last card.Was the last card supposed to be discarded?
In my experience, the dealer did not make a mistake here. The players must ask the dealer to run it more than once (provided that the casino allows the players to run it more than once). If, however, wherever you are this is considered a mistake, usually instead of discarding the last card, the dealer simply deals another card for the second run.
The dealer is not supposed to ask if they want to run it twice!!
If they want to run it twice it is the players obligation to ask. It is one of those things that is totally optional. Not the dealers business to get involved. Some players like to run it twice some do not. If the players are not talking about running it twice, its time for the dealer to tap the table once and burn and turn. It like asking a player if they want to straddle a blind. Some things for dealers are not to be done, asking players if they want to run it twice out of the blue is one of them.