To reproduce:
1. Open project
2. Open and play 'AnimatorControllerPlayable' scene
3. Choose 'char_ethan' game object and enable 'B transfer' in 'Playable handler' script in editor

Expected: It should transfer to 'PlantNTurnLeft180' animation
Actual: character 'sits weirdly' and error is thrown: 'Cannot connect output 0, it is already connected, the tree topology will be invalid. Disconnect it first
UnityEngine.Experimental.Director.PlayableGraph:Connect(PlayableHandle, Int32, PlayableHandle, Int32)
CrossFadeMixer:Crossfade(AnimationPlayable, Single) (at Assets/CrossFadeMixer.cs:52)
PlayableHandler:Update() (at Assets/PlayableHandler.cs:63)'

Reproduced on: 5.5.2p4, 5.6.0f2, 17.1.0a5