While playing Murder Mystery, I had activated a quest that made me get 60 coins in MM to get 20 quest points. After playing a game, it had said I had collected 30/60 coins, though I had only collected around 15. This confirmed my theory when I picked up a coin and it updated the quest 2 times, making the game think I had collected 60 coins even though I had only collected 30. It wasn’t because I was picking up epic coins or super coins, because it updated twice when I picked up a normal coin. To recreate this, one just needs to activate a quest that has to do with picking up coins in MM and closely monitor the amount of coins you pick up and the amount of coins the game says you picked up. I was playing on a Nintendo Switch on the North American region at the time, but this may have been a one time glitch, not too sure though.
Hey there 
Thank you for submitting a bug report. This issue has already been reported and logged.
Have a great day! 