The jumpscare of the animatronic must happen before you can death coin them, so, for example, you have to get jumpscared by Toy Freddy first then you can Death Coin him. If he gets re-added by DD then you have to get his jumpscare again before you can use the Death Coin on him again. This applies to all of the animatronics.

Sorry I didn't make this as clear as I should've in the first place.

There's no limit on how many times you can use the Death Coin, you can use it as many times as you want on whoever is susceptible. But, if DD adds a character you have to get their jumpscare before you can Death Coin them. There's only 4 characters that DD can add that can also be Death Coined, so you wont be able to do this often. If you're having trouble with DD, I would reccommend both being aware of animatronics who can end the night before DD has time to add an animatronic, and to make more use of DD repels.