Question: What if the identical tiles is discarded in rapid succession and it is someone’s Mah Jongg tile?

Answer: If two identical tiles are discarded, one right after the other, and someone calls the tile for Mah Jongg, it is the 2nd discarded tile that can be claimed. Therefore, the player who discarded the second identical tile pays double the hand’s value shown on the card. 

Source:  Mah Jongg Made Easy 2024, page 17, #9