Why is the object on the right being selected when I select the object on the left? I need to understand the reason of a specific behaviour. I'm in Object mode and when I select one feather on the left wing, automatically the corresponding feather on the right wing lights up, BUT I have no Mirror modifier, no X mirror, no Topology mirror activated. Do you know if there are others reason that explain this behaviour? 

Check the attached video below:

attached video
At the very beginning of your movie, while you're still in Edit Mode you can see that both feathers are actually one object with two unconnected mesh parts.



When you select one of the feathers, the other one is highlited as well because it is the same object.