My supplier's system sends me automated invoices by email, however, due to a glitch with their system, this email gets sent to me twice, a few hours apart. Since it is sent twice, my Gmail does not detect it as a duplicate, so it appears in my Gmail inbox twice. Both of the emails I receive have the same subject, message body and attachment. I have a mail filter rule setup to forward these invoice emails to a 3rd party.Is there a way to stop the 3rd party from receiving the both identical copies of the invoice? For example, is it possible to set filter conditions to only forward once per subject line (e.g., subject line must be unique). 
Only forward an email once per subject line
gmail;gmail filters
Currently, there's no way of doing this in GMail alone. I'm filters, each email (not each conversation) is processed individually, and so you don't have any option to operate differently depending on the existence of a previous email.However, I'd argue that doing so is a bad idea. At the moment, you are trusting that such emails are exact duplicates, but nothing prevents the sender to send emails with the same subject and different content. If a third party is in charge of paying those invoices for you, you definitely want to keep them updated with exactly was has been sent. This intelligence is way out of the league of GMails filters.