What do you call someone who is seemingly working with you but is actually working against you?
closed as general reference by coleopterist, J.R., Andrew Leach, tchrist, MετάEd Mar 14 '13 at 15:44
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Depending on the situation backstabber may be an appropriate word. For example, someone may ask the advice of a friend regarding a love interest whilst that same friend has ideas of his/her own regarding the same person. The friend may seem to give good advice but is in fact trying to make the situation better for him/herself.