Invalid Session

When trying to log into a minecraft server, I get an invalid session error. I've pinned this down to being logged into both the minecraft launcher and minecraft.net at a time. (I usually will be logged into both when working with skins.) But I sometimes get this problem for seemingly no reason at all, other than having taken a while between restarting minecraft. This is usually fixed either by closing minecraft and restarting my launcher, but this process usually takes over three minutes between getting the error and getting back into a server. This problem is very annoying, and I would like something to be done about it, if possible.
Thanks!