I actually had one case of a civ competing with me for a diplo victory (maybe because of the last patch/es), but I may have been a coincidence. Hiawatha and I were the two superpowers, late game (we had each gobled 2.5 other civs), we were the two last civs owning their original capital city, but we were best buddies since the beginning (liberated civilian, warring the same enemies, friendship, etc), so there was no tension between us (I could have theoretically pulled off a domination win, since the tactical AI is nothing to be afraid of, but it would have been too much of a world war, and I wasn't in the mood for it [plus, it would be WRONG]).
The thing is, I wanted a time victory, but I had allied all the CS, so I didn't build the UN. Hiawatha eventually did (maybe he was afraid of me winning that way?). And ever since he did, he bought one CS out of me every turn, since I needed all the CS to get the diplo victory, and as such there were 2 failed votes in a row. I eventually won by pouring 20k gold spread over the 6 CS (he could have gotten one CS if the had payed a single CS 3 or 4k gold, but I guess he stopped trying after paying 2k).
If he tried to avoid my victory by bribing the CS, that's good programming; if he just wanted the bonus of some of the CS, was trying to get the diplo victory himself, or just didn't know what to do with his 17k gold (!), it's meh programming.