Steps to reproduce:
1. Download and open attached project.
2. Notice that the "Console" window outputs this message twice:
"NewBehaviourScript cctor()
UnityEngine.Debug:Log(Object)".
It indicates that the constructor of "NewBehaviourScript" object is called twice.
3. Re-import all the assets or enter the Play mode.
4. Notice that this message is output once.

Expected result: when the Editor starts the constructor of the object should be called once.

Actual result: when the Editor starts the constructor of the object is called twice.

Note: with some versions of Unity the constructor of the object is called three times when the Editor opens.

Reproduced with:
5.3.2f1, 5.3.4p6, 5.3.5f1, 5.4.0b18.