Double Posts
Doubled chat posts, doubled transactions, or other instances of doubling are caused by some kind of interference with your connection.
1) The problem mostly commonly has been caused by Norton Antivirus:
Uninstall Norton per https://support.norton.com/sp/en/us/home/current/solutions/kb20080710133834EN_EndUserProfile_en_us then reinstall. (Refer to SL BUG-11891.)
2) The problem has also been traced to Comodo Antivirus:
Test by turning off Comodo then rebooting your computer. See if the problem stops. If it does, you may need to use a different antivirus, or contact Comodo about the problem.
3) Sometimes this is a temporary network glitch:
Reboot your modem/router: Unplug them from all power and leave them unplugged for at least two minutes. Then reboot your computer too.
4) If you are on wifi, try wiring up to your router to see if that fixes it.
5) Doubled interactions recently attributed to ExitLag. Refer to FIRE-28965 Viewer Interactions Doubled