I don't think it is very logical for top5 monthly and top 5 all time members to be merged when awarding the prize of free month elite membership. My reasoning being you can assume and see that the top all time members are most often also the top monthly members so why not just make the categories separate.
Award the top 5 all time members free monthly memberships because they are loyal and frequent posters. Award the top 5 monthly members separately to give more incentive to users to go for it.
To summarise-- if you're already in the top 5 all time you should not be included in the monthly prize list. - 10 individual memberships should be awarded with no overlap. Just my thoughts improving the system.