You replaced a seal, or a gasket? Big difference. If you replaced a seal, which one? There are several.
If you replaced the gasket on the rear cover and it leaks.... then no, you can't use the gasket over. If it leaks, then taking it off and putting it back on isn't going to fix it.
You can drain the rear into a clean pan and use the oil again. If you get dirt in the pan of oil, use cheesecloth or a paint filter to strain out the dirt. Then install the rear cover using black RTV with no gasket. Install the RTV on the rear, making sure to wipe the mating surfaces down with brake cleaner. Install the rear cover and torque the bolts to your liking. I just make them tight. It's not a head gasket, so torque is really unimportant.