Reproduction steps:
1) Open attached project.
2) Open "New Scene" and go into playmode.
3) Without exiting playmode manually, go into "New Scene1" and enter playmode.

Expected result: "New Scene" should not be loaded on top of "New Scene1".
Actual result: In the hierarchy you can see that "New Scene" was loaded on top of "New Scene1"

Workaround: Exit playmode before switching scenes in the Editor.

Regression introduced in: 5.3.1p4