From http://blog.360.yahoo.com/y_groups_team, yahoogroups gives their comment about duplicate messages and e-Mail delivery. Many people already asking about this trouble in yahoogroups. So, for more information, they give some explanation about this.

Here are their comment. “We’re aware of the duplicate message bug that has been affecting groups today and are working to resolve the issue (a side effect of this bug is that some messages are also being delayed). We have actually pushed two fixes already, but we are aware that some groups are still experiencing the problem and that more needs
to be done to fully resolve the issue.”

There is, however, one silver lining to this bug. It was the result of their latest system updates intended to improve e-Mail delivery speeds. So once the bugs are resolved, they should see a significant reduction in the time it takes to deliver messages to Yahoo! Groups members.

The team said that they will update this blog entry as soon as they have more definitive
information on the status of this issue.