What happens if you try to jailbreak a jailbroken iphone?

Yes, I know it would still be jailbroken but I wanna know what happens in the process of jailbreaking it again

Added (1). yes! That is precisely what I want to know! Will there be a notification saying I already downloaded cydia?

Nothing would happen it would probably remain the same and/or give some sort of notification that you've already downloaded the app cydia.

It voids the manufacturer warranty