In the attached project there is a Canvas set to Screen Space - Camera rendering mode with the Main Camera selected as the camera to render to. There are two CanvasRenderers as children of the Canvas. The Main Camera has a script (Rotator.cs) attached that rotates the camera every frame in Update(). See in the profiler that CanvasRenderer.OnTransformChanged is most of the time called twice per CanvasRenderer instead of once.

Also see attached image: OnTransformChangedProfilerPic.png

To repro:
1) Open attached project
2) Run in Editor or build to standalone
3) Attach profiler
4) See in profiler that CanvasRenderer.OnTransformChanged is most of the time called twice per CanvasRenderer in the Scene

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Fixed also in 5.6.1p1, 5.5.3p4 & 5.4.5p2