I have nightmares with this issue too...
I belive I'm correct in saying the frequencies of drum hits are not stable and change over time in the decay, so if you double two drum sounds when they are in phase on the attack they will very likely go out of phase during the tail. Shift the pitch of the samples to complement the original drum (flip phase if necessary), and either cut the tails off the samples, or gate the tails off the original. Try eq-ing out problem frequencies if necessary.
Good luck!