Hi Nick,
you are going well. I figure because of the depth of the housing the seal fits in I figure there is the bearing in there and probably 3 seals behind it. They possibly figured they could use 3 seals as a good seal and bearing retainer in one. Anyway just replacing the outer seal should keep the oil in for many years. Rubber grease is good if you have some, otherwise a wipe of oil will do the job. Also a wipe of silicone around the outside of the seal helps it go in and helps seal it against leaks there
