To reproduce:
1. Open the user attached project
2. Open the SampleScene
3. Enter the Play Mode
4. Select the Robot.
5. Click Toggle in the Inspector window under Script Component few times.

Actual results: AnimationPlayableUtilities.Play adds count existing playables instead of replacing them
Expected results: AnimationPlayableUtilities.Play replace existing playables

Reproducible with: 2017.1.5f1, 2017.2.3p4, 2017.4.10f1, 2018.1.9f2, 2018.2.7f1, 2018.3.0b1

Notes:
- Each click triggers the OnValidateMethod which calls AnimationPlayableUtilities.Play.
- Each call increases the Clip Count and other details shown on the Animator, indicating that the existing data is being added to rather than replaced.
- This seems to have no effect on gameplay or anything else, but when the numbers get sufficiently large (over 1000) it starts affecting the frame rate.
- There doesn't seem to be a way to check if the PlayableGraph is currently being played by the Animator in order to avoid calling Play repeatedly.