It's DAMMIT not DAMNIT, DAMMIT!
See, it's like this. If you want to say damn then you say damn, not dam, unless you want to talk about a dam. If an object is there, and you do not care for it, for whatever reason, and you somehow wish damnation upon said object, then you damn it. However, let's suppose you smack your finger with a damn hammer, then you say dammit! If you want that hammer to go to hell, then you damn it. Never, ever, never, do you "damnit". That's just stupid.
When I see "damnit", I pronounce it dam-nit. Because why not? When I see dammit, then I say dammit, and I mean it.