Let say someone have a theme and a plugin, which both are requiring BudyPress plugin using TGMPA, both of them should have their own copy of TGMPA and will include the TGMPA class on their own.

This will cause `new TGM_Plugin_Activation` called twice, therefore all the hooks and actions are called twice.

And as the `register` function doesn't check for registered plugin(s), the BudyPress plugin will be registered twice also.

So 2 `BudyPress` registered in the `plugins` array, processed twice because of the previous issue with class instantiation, will produce 2 x 2 number of notices:

http://prntscr.com/1nv19c

With the changes, those issue should be fixed, and only 1 notice produced.