How to reproduce:
1. Open attached project "case_1115985-PhysicsDebuggingSceneViewUI" (This is just an empty project)
2. Make sure the Physics Debug window is tabbed next to another window e.g. the Inspector window
3. Untab the other window and position it elsewhere
4. Move the untabbed window back next to the Physics Debug window
5. Observe Scene view

Expected result: There is a single Physics Debug overlay panel in the Scene view
Actual result: another Physics Debug UI box appears on top of the first in the Scene view

Reproducible with: 2017.4.18f1, 2018.3.2f1, 2019.1.0a14, 2019.2.0a1

Note: if steps 3-4 are repeated more Physics Debug overlay panels will stack up vertically in the Scene View