I've had this idea for a year or so and I find it hard to believe that I am the only one to have ever of thought of this. This feature is not available in any web-based or email client that I know of. I would love to see this for Gmail. I posted a topic on the Gmail help forums but never received any meaningful replies.
A lot of people forward on emails to groups of other people, usually of the fun or joke variety. I don't want to be that guy that forwards on the email to my "fun" group of addresses only to send it right back to the person that just sent it to me or to someone that may have already received it previously in the thread of messages. I find myself hitting the "forward" button, adding a group to the recipients list, then having to go through the list and remove people that received the email just like me.
So, why not have a "Forward without Duplicates" button that parses through the message body and remove any address found from the recipients list. This way, anyone that has already been included in the email thread does not receive the message again. I don't forward on emails anymore b/c of the annoyance of having to add a group of addresses and then slowly and painfully go through and remove each one that does not need to be on it.
I have tried using firebug to isolate what needs to be parsed through, but it is tough. Does anyone have any direction they could point me in? The Better Gmail extension for Firefox includes some scripts that have been written, but I have no idea where to even start to be honest. Is there a way to look through these scripts to use as learning examples?