I'm not sure a single extra pill will kill you but it will probably lower your BP quite a bit. And how much time has passed between one dose and the other? Are you talking about one in the morning and then maybe another at night because you don't remember or taking 2 in the morning because you're in a hurry?
If you're not having any symptoms, that's good. But really - you should think about getting a pill organizer box if this is hard for you. Better safe than sorry. And next time you're not sure, just call poison control.
An overdose of diltiazem can be fatal, so if you are not sure if you missed a dose, to be safe, don't take any that day.
I used to get a marker and put M T W T F S S above each pill in line (they were in the foil strips). That way I could see if I had taken that days medication.