Following are the main story arcs regarding Buffy's struggle with herself. Well, sorta. More like...Buffy versus The Slayers! These arcs have more to do with Buffy's personal issues as the Slayer and are central to understanding just what's up with her.
That aside, Buffy feels freaky when she realizes that Kendra exists as a slayer only because of her death by The Master's hand in Prophecy Girl.
Couple in the fact that Giles takes to Kendra, who does things very much by the book -- the Slayer Handbook, that is -- and Buffy becomes well, if not slightly jealous, then maybe feeling a little inadequate. Buffy and Kendra don't seem to work well as a team at first, but later they're more in sync, when they're taking down Spike, Drusilla, and The Order of Turaka. So while Buffy and Kendra may have very different temperaments, they're able to learn from each other some, and in the end, it makes Buffy feel better not being "..da only freak anymore."
Buffy and Kendra meet up again in Becoming part 1. Sure, they exchange some remarks about beating each other up, but it's obvious that Buffy is glad to see Kendra. Well, except for that part about her reason for visiting Sunnydale again..."..a very dark power is about to rise in Sunnydale." They bond a bit more, and Kendra gives Buffy her lucky stake, Mr. Pointy. Buffy promises to get Kendra a stuffed animal, too. But the thing here is, Kendra dies and Buffy is the one to find her. This brings up a whole lot more issues with Buffy...it also must be freaky knowing that you're still alive, while the Slayer who was activated after your own death is...well, dead. I'd say that the whole Kendra/Buffy relationship was a good one, overall.
More 611! (for Buffy vs. Faith)
The ABCs of Sunnydale