How to reproduce:
1. Create new empty project
2. Create UI -> Canvas
3. Create new C# Script in Project Window and open it
4. Enter Play Mode in Editor and select Canvas in Hierarchy window
5. Edit script, save it, go back to Unity (make sure Canvas is selected) and wait till script recompiles
6. Do 4-5 steps few times

Expected result: Warning message appears one time if script saved
Actual result: Warning message appears one time in the first time and doubles itself in the second time. Every time you do 4-5 step, warning message doubles. This error will appear on every recompile until Unity is restarted. Doubleclicking on the warning doesn't cause anything to get selected as it's not caused by a scene object.

Reproduced with: 5.5.4p3, 5.6.3p4, 2017.1.1p3, 2017.2.0f1, 2017.3.0b2