Steps to reproduce:

1) Download attached project "Projector Bug.zip" and open in Unity.
2) In Hierarchy tab, click on Should Be Ignored By Projector and in Inspector check if Layer set on "UI".
3) In Hierarchy tab, clock on BlobShadowProjector and check in Inspector tab if UI layer is ignored by BlobShadowProjector (Ignore Layers).
4) Enable Frame Debug and you will notice that once Should Be Ignored By Projector enters Main Camera's field of view, Cube being rendered second time(see attached image "2ndtime.png"), even Should Be Ignored By Projector is set on layer "UI" which is ignored by BlobShadowProjector.

Expected result: Cube should't be rendered 2 times.

Actual result : Cube being rendered 2 times
even Should Be Ignored By Projector is set on layer "UI" which is ignored by BlobShadowProjector.

Tested versions: 5.2.4f1, 5.3.5p4, 5.4.0b22