Have you and a best friend already liked the same girl/boy before?

No, that hasn't happened as me and my best friend have quite different preference in women. Or more like his preference is so specific, it's "left" me so broad an avenue, so there has been no clash of interests.

No. Thankfully we have different tastes in guys

Yes,me and my friend both liked the same guy.We were both honest about this and basically just agreed to let what happens happen and have no rough feeling about it.You just have to be honest about it and not sneak around with it.

This only happened to me a couple times.
The first time was in high school. It became really complicated - she liked him and thought that he liked her, but he liked me and asked me out. I didn't realize that she liked him at that point. She hadn't told anyone. We weren't as close then as we became later. She was really upset by it, and then after he and I broke up the next year, they ended up dating but he and I were still friends - she wasn't really able to deal with that because she always had that insecurity that I was the one he really loved. I'm not going to say either of us just should have forgotten about it, but it did make things rough for a bit.

Then the second one was kind of... weird. She was insanely jealous... but of both of us. She'd talk to me about how my boyfriend was so wonderful and if he left me that they'd end up together. I was like "Um... really...? You need to say this time?" Then she'd get weirdly jealous that he was with me, in the sense of she'd tell people she wanted me to herself and it wasn't fair that he was stealing me away. We didn't stay friends, but that wasn't really the main reason for it. As people, we just weren't compatible. I suppose that should have been a red flag though. I still don't quite get it.