How to filter to show only one message from a specific sender/subject

[Brian]

I would like to filter out all but one email with the same subject
line from a specific sender.

I subscribe to a message board that sends an email every time a
posting is made to a thread, instead of just the first posting after I
have read the thread. Is there a way for filter out the duplicate
messages and just keep the oldest email?

Thanks,
Brian