Dupplicate symbols errors when adding OpenKit - objective-c

Question

I'm trying to add a leaderboard to my game by using OpenKit Framework. Following their documentation, I've added all the neccessay libs + framwork but the following errors occured :
duplicate symbol _OBJC_IVAR_$_AFOAuth1Client._applicationLaunchNotificationObserver in:
/Users/Sa7bi/Library/Developer/Xcode/DerivedData/helloworld-erlcedkitaorqobgauzlzyvacnfi/Build/Intermediates/helloworld.build/Debug-iphoneos/helloworld.build/Objects-normal/armv7s/AFOAuth1Client.o
/Users/Sa7bi/Dropbox/Aliens Wings/HelloWorld/OpenKitSDK/libOpenKit.a(AFOAuth1Client.o)
duplicate symbol _OBJC_IVAR_$_AFOAuth1Client._key in:
/Users/Sa7bi/Library/Developer/Xcode/DerivedData/helloworld-erlcedkitaorqobgauzlzyvacnfi/Build/Intermediates/helloworld.build/Debug-iphoneos/helloworld.build/Objects-normal/armv7s/AFOAuth1Client.o
/Users/Sa7bi/Dropbox/Aliens Wings/HelloWorld/OpenKitSDK/libOpenKit.a(AFOAuth1Client.o)
duplicate symbol _OBJC_IVAR_$_AFOAuth1Client._secret in:
/Users/Sa7bi/Library/Developer/Xcode/DerivedData/helloworld-erlcedkitaorqobgauzlzyvacnfi/Build/Intermediates/helloworld.build/Debug-iphoneos/helloworld.build/Objects-normal/armv7s/AFOAuth1Client.o
/Users/Sa7bi/Dropbox/Aliens Wings/HelloWorld/OpenKitSDK/libOpenKit.a(AFOAuth1Client.o)
duplicate symbol _OBJC_IVAR_$_AFOAuth1Client._signatureMethod in:
/Users/Sa7bi/Library/Developer/Xcode/DerivedData/helloworld-erlcedkitaorqobgauzlzyvacnfi/Build/Intermediates/helloworld.build/Debug-iphoneos/helloworld.build/Objects-normal/armv7s/AFOAuth1Client.o
/Users/Sa7bi/Dropbox/Aliens Wings/HelloWorld/OpenKitSDK/libOpenKit.a(AFOAuth1Client.o)
duplicate symbol _OBJC_IVAR_$_AFOAuth1Client._realm in:
/Users/Sa7bi/Library/Developer/Xcode/DerivedData/helloworld-erlcedkitaorqobgauzlzyvacnfi/Build/Intermediates/helloworld.build/Debug-iphoneos/helloworld.build/Objects-normal/armv7s/AFOAuth1Client.o
/Users/Sa7bi/Dropbox/Aliens Wings/HelloWorld/OpenKitSDK/libOpenKit.a(AFOAuth1Client.o)
....
ld: 19 duplicate symbols for architecture armv7s
While searching on this website, some users suggested the following:
Compilation of the same file more than once ==> checked my [Build Phases | Compile Sources] nothing
Importing ".m" files instead of ".h" ==> checked all my imports, nothing
I really have no idea what is causing this.
Update: I have forget to mention that Google Ads SDK is included in my projet. When I delete the "-ObjC" linker flag, OpenKit works fine but Google doesn't.

Answer 1

After many weeks of search, I've found a solution. Instead of using the "-ObjC" linker flag for Google Ads, "force_load" was the perfect solution.
The "-ObjC" flag was messing with the OpenKit lib, according to the folks #OpenKit:
The problem is that the -ObjC flag loads all categories, and we removed some categories from the OpenKit SDK to make it work without using that flag.
force_load :load only that framework without messing with anything else.[source]