Is it ok to say yes to a guy who courted your friend?

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I have this guy friend who is courting me now.We used to work in the same company but i left the company after few months.We started going out and then he proposed to me.He is still working in the same company with my friend whom he used to court.I am beginning to like him but I am afraid that they might have a relationship behind my back.My friend got him first in the first place right?so now,even though I like him,I can't say "yes" to him...fearing I might end up hurting..what's the best thing to do?