I definitely consider this bad form and it has been explicitly stated somewhere(maybe a message from Mike or even in the rules) not to do it. You could wind up with 2 trades of the same player ausing one of the mangers to be dropped. What if they signed another trade because they rightfully believed they'd acquired your player ? Bad situation for a lot of people.
If you want to write multiple e-mails to offer a guy that's OK. But sending trades to be signed with the same players is a mess waiting to happen.

While I'm here let me ask a trading question. I've received a few trade offers that make it obvious that the other manager never looked at what I might need. I had a guy offer me upgrades in LF & RF for Vina. My RF was Vlad and the only LF better than mine was Luis Gonzalez; was he really offering Luis for Vina ? No, he just hadn't even looked to see if the "upgrades" were better than what I had. Another team offered me a young 3B for what he wanted when I already have Aramis Ramirez & Beltre. Look before you offer, it'll make things better.