If all seems physically OK in the seal area, my guess is that you are building crankcase pressure. Does that seem to be the case? That is if it isn't tearing the seal from a mar on the crank. Does the seal look stressed from what you can tell after removal?Is it being installed at the proper depth and not partially over an oil passage? I have not had that experience nor do I really work on many Command horizontals. I've had issues on other horizontals that had other external leaks and would actually blow oil though. As an example, a Briggs with a missing dipstick seal. So if you are building pressure, a likely failure would be a head gasket or a breather issue. I do remember though that the horizontal's breather reed is under the breather cover. I would assume that there is a drain hole near the bottom of the breather chamber too. So, depending on engine condition that gives you some things to consider. You may want to even try a new oil filter (OEM) and oil (10W30) for starters if you haven't so far.

It doesn't seem to be over pressured. The end of the crank looks alright and the seal looks perfectly fine

what weight of oil are you running in that engine ?