I was a little too keen and eager previously and sent my sample chapters off too soon. Since been rejected, understandably. I totally underestimated the editing process. I have now redrafted the entire story and was wondering is it frowned upon to send it again to the same agents?
I'm not an agent, so I'm just guessing, but I wonder how many agents would even notice if they received the same title from the same author - I imagine they are inundated with manuscripts and don't have time to keep records of who sent what and when.
That said (or written, if I'm being pedantic), I would always check the website to see if they have their own rules about resubmissions. Agencies that take only email submissions might have an easier way of keeping track if you've contacted them before.
Also, there are lots of agents - is there a particular reason why you want to send to the same agents again? (That's brought a question of my own to mind.)