A player just threw a joker and named it 'flower', which is the same as the previously discarded tile. Another player called the 'flower'. What happens now?

Nothing happens, as the joker can not be picked up. When discarding a joker it can either be named 'joker', 'same', or the name of the previously discarded tile. It is not possible to call a discarded joker as it is considered 'dead'. It is a player's responsibility to watch and listen, and when doing so a player will know that a joker has been discarded and no calling is possible.