Three things to look at before you start replacing the main seal again, is the oil leaking out of the side seals? If not, is it leaking from above the crankshaft draining down onto the crankshaft and getting slung out from there making it looks like the seal is leaking or is it in fact leaking out of the main seal itself, or maybe the oil pan gasket above the main seal
I've seen more than one main seal get replaced when it wasn't leaking to start with
Try squirting brake cleaner all around the rear of the seal holder and the back of the crankshaft flange that faces the block so it is really clean and put it up high enough to look at while the motor runs with the dust cover off the tranny or bell housing to help you see where the oil is actually leaking from
I've seen main seals replaced when the oil was leaking from the rocker arm covers on B,RB and 426 Hemi motors more than once