Commentary for Kiddushin 122:9

God when speaking to Cain uses a double formulation. This accords well with R. Meir. R. Hanina b. Gamaliel would say that without the double formulation we might have thought that if Cain does not act well he would be neither rewarded nor punished. The double formulation teaches us that if he sins, he will be punished (sin crouches at the door, always reminds me of my dog wanting to go for a walk).