The doubled impression on the reverse won't make much difference to the value.
This coin just happens to be very nice one. Coins of Severus Alexander often turn up in nice condition, I have a Severus Alexander myself, but this is one of the best I have seen.
With this coin, the lower die is fixed, and the metal is struck by the reverse die. In this case, the coiner has struck the reverse twice. The reverse die has bounced before being struck again. The slight doubling as seen would not have even been considered as an error by the coiner. Partial doubling does not occur on ancient coins, because they never used working dies or hubs, in the way modern coins are made.