Compatibilists about mental causation maintain that every effect of a mental cause also has a sufficient physical cause. This, many object, looks suspiciously like overdetermination. In response, compatibilists argue that there is a key difference between overdetermination and compatibilist mental causation: While the overdetermining causes in genuine overdetermination are independent, the causes in compatibilist mental causation are grounding-related. For this reason, they cannot overdetermine their effects.

In this paper, I argue that, to the contrary, grounding-related causes can sometimes overdetermine their effects. I present a novel counterexample, which arguably qualfies as a case of grounding-related causes overdetermining an effect. I show how this counterexample refutes two proposed necessary conditions on overdetermination, by Kroedel and Schulz (2016), and Stenwall (2021). Finally, I suggest a way to modify Stenwall’s proposal such that it no longer misclassifies the counterexample.