Sounds to me like someones replaced it before, and not been careful about how they used a screwdriver to remove the leaky seal. Even a small scratch will quickly chew out a seal. Also make sure that you aren't putting too much oil in the box either.

Check the seal for any unusual wear. If it continues to leak, use a thicker grade gear oil.

If the surface on the output shaft is scratched, use some 1200 grit wet and dry on the surface. Also make sure that you are using the right seal part. Maybe they are selling you one that's close...but not quite the correct size.

Chris