Multiple invites from iCal with Exchange

I have a corporate Exchange account and I use my Macbook to connect to it. iCal is synchronised with the Exchange account and I send meeting invites to my team. I explicitly list the email addresses on the invite (I don't use mailing lists).

One annoying effect my co-workers have reported to me is that "I" keep sending the same invitation several times. I've checked that I "did" send the emails (they have shown me their mailbox), but I don't know what have I done to trigger that. I haven't updated the meeting whatsoever. The only thing that seems to trigger that is the 15 minute reminder I have on most of my meetings. Though, invites have been re-sent days in advance, even when the reminder hasn't gone off.

Can you help me please?