Apple Mach-O Linker Error(Duplicate Symbols For Architecture armv7)

 duplicate symbol _OBJC_METACLASS_$_BFAppLinkReturnToRefererView in:
     /Users/User/Documents/FacebookSDK/FacebookSDK.framework/FacebookSDK(BFAppLinkReturnToRefererView.o)
     /Users/User/Desktop/Project/Project/Project/Assets/Facebook/Editor/iOS/FacebookSDK/FacebookSDK.a(BFAppLinkReturnToRefererView.o)
 ld: 889 duplicate symbols for architecture armv7
 clang: error: linker command failed with exit code 1 (use -v to see invocation)

I have searched to find the solution of this error but i couldn't. There are tons of similar error like this, but this one hasn't been asked by someone yet. This problem shows up when i add "-ObjC" to the "Other Linker Flags". I need to add it to use Admob service. What could be the problem? How can i solve this?