Is it fair to get in a relationship when you still like somebody else?

I'll keep this short, I've liked this person for around a year-ish but like nothing will happen ever ever. She knows how I feel and that's that. I'm attempting to get over her and I'm getting there but she's still in my mind a lot. Anyway, I'm talking to this new girl and she's cool, we clicked straight away, she really likes me and we're seeing each other next week and it's likely it COULD blossom into a relationship. Now i'd be happy in a relationship with her but is it fair on her if I still like this other person? What should I do?

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