Ok, an update:

It appears Gmail doesn't provide any way to filter those messages.

Gmail allows filtering based on three headers only:

1. From
2. To
3. Subject

Beyond those headers, Gmail only allows to filter by positive and
negative text search on the message body.

Since the copies sent by ruby-talk are identical to the original sent
message in all the above 3 headers, and also of course the body,
there's no way to distinguish and selectively remove copies by using
the current Gmail filters.

So it appears the only way to avoid message duplication if you have
Gmail is by using a more sophisticated email client and accessing
through POP3. Which is too bad, since the web client is so much more
comfortable and portable.

Alder