Arduino IDE v1.8.4 on Sierra 10.12.6 (OS version didn't change, just IDE upgrade).
This happens often and is quite annoying.
When I copy a text from anywhere (IDE, Notes, Safari) and paste it to IDE.. it pastes twice.
I'm using cmd+C, cmd+V
It happens once in 3-5 minutes, but behavior continues for aprox. 30 sec.:
Text pasted to other apps is pasted once, in IDE - twice.