What do I do if he likes me, but I'm obviously his second choice?

I'm kind of confused. There's this guy that I like and he likes me. The problem is, I'm his second choice. He's in love with my best friend. He can't have her though because she's dating his twin brother. What do I do? Why should I chase after someone who likes my best friend more than me? If he ever gets with me, how do I know he's not using me to try to get my best friend?
What do I do if he likes me, but I'm obviously his second choice?
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