I thought it was possible to send a hero in ahead first to attack camps, then follow it up with troops. Now the message "You already have troops on the way there" appears and prevents you from sending a second attack while the first one is on its way. Is this new or a bug? Or maybe it always was like that and I forgot.
From my experience, you can send multiple attacks at hideouts, but only one attack at a time can be inbound on camps. I assume the latter is because those are visible to all the players in the kingdom, whereas hideouts only appear to you.
^ ^ What he said.
Robber Hideouts are player account only and you can mess them up any way you see fit.
Robber Camps are Kingdom wide and you only get one visit per camp. 'Defeat the Robbers in the Kingdom' will be the message when they appear, usually twice a week.