[Fix] multiple purchase of the same mount from store. (#259)

When buying a mount, it was possible to buy it indefinitely, causing the player to lose coins.

data/modules/scripts/gamestore/init.lua

@@ -1416,6 +1416,10 @@ function GameStore.processOutfitPurchase(player, offerSexIdTable, addon)
 end
 
 function GameStore.processMountPurchase(player, offerId)
+	if player:hasMount(offerId) then
+		return error({code = 0, message = "You already own this mount."})
+	end
+
 	player:addMount(offerId)
 end