If the friend you like doesn't have a BF/GF, ask them out. If they say yes, congratulations (make sure they know it is a date and not just friends hanging out!), if they say no, lose contact with them.
If the friend you like has a SO, lose contact with them, no matter what.
It is that simple folks, life is too short to be getting stuck in friendzones. You might be saying "but he/she shows signs of liking me back" if so, then they'll say yes to a date. You can only blame yourself for being in the friendzone, even if the other person is manipulative. 
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