I'm unsure if I'm getting requests to join my Alliance from players flooding alliances with requests or the game allows players to request to join alliances while they're in an alliance. But, it would be nice if we could invite players who are already in alliances, as it gets frustrating when players request to join and when you invite them, the game tells you they're already in an alliance.
If the game allows players in an alliance to send a request while being in an alliance, this is an oversight.