it's actually more of a morality/ethics deal for me; I'd be going by this rule even if you didn't have it. I follow three posting rules at all times
1: Wait three posts IC before you post again (only applicable in RPs with four or more people, obviously)
2: Necroposting is only to be done on threads you own while they're still on the first page. Even in this scenario, apologize to the mods.
3: Never, NEVER double-post. My strictest rule is also the one I break the most T^T