There seems to be a Google Talk problem affecting some users on the chat client in Gmail's interface.

The problem manifests when someone uses Google Talk to chat -- each message seems to be duplicated and sent to another contact in the user's list as well.
This seems to affect users on Google Talk who are not currently using Hangouts, Google's replacement for Google Talk.
The issue seems to be affecting users from all parts of the world and we have personally experienced the issue and also getting similar reports of it affecting other users from various websites such as theverge.com as well.
We've reached out to Google Malaysia for comment but we are still waiting for an answer.
Update: Google's App Status Dashboard now reports that the Google Talk issue has been resolved and services are gradually returning to normal.
Update 2: The App Status Dashboard was updated at 10.15pm: "We are continuing to investigate this issue. We will provide an update by 9/27/13 12:00 AM detailing when we expect to resolve the problem.
Earlier today we identified an issue with Google Talk messages sent between 22:00 PST and 01:30 PST. Any messages sent after 01:30 PST are unaffected."