Packages conflict

I have Unity 5.0.2f1. Firstly, I've successfully added GooglePlayGamesPlugin-0.9.20.unitypackage to my project. Then, I've tried to import GoogleMobileAds.unitypackage, but I only got these errors:

 Error importing folder (The pathName assets/plugins is already mapped to fce8a713f1e5a4cc4b9973d1ef630f31. But the meta data wants it to be mapped to cbde64d36fd994c458fffca9e931b232)
 Error importing folder (The pathName assets/plugins/android is already mapped to b8f0d9a6a7f9240c981894807effddbc. But the meta data wants it to be mapped to 2f5d736f7c4cb4c1e80d0816d0e81625)
 Error importing folder (The pathName assets/plugins/ios is already mapped to 6490bb8acab6f4f92b29615e7429b8df. But the meta data wants it to be mapped to da135550add3c4abca622bda5280d204)

How could I cure this?