No in either situation below:
Completing an offer you already did on CashCow
Each offer can only be completed once per user. If you've already done it, you won't be rewarded for doing it again.
Completing an offer on CashCow that you already did on another platform
The same rule applies. Our partners (typically app and game developers) put their offers on multiple reward platforms to reach as many new users as possible. If you've already completed an offer anywhere, your install is already registered and you won't be tracked as a new user on CashCow.