new email ID created if account exists through different site

on my site I have allowed to login through
1)custom sign up form
2) google
3) Facebook
all work correctly. But if I create a account through fb (or google or custom sign) up my email gets registered !!!
but next time if i try to login through different service then new account is created but i want it to be directly merged to account existing !!! merge option is given bt it should happen automatically!!! how do I do it???

Make setting:
Confirm account ownership -> no
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