From the limitations of IMSMO documentation:
If you enable the Save copies of messages in the Sent Items folder setting, duplicate messages are saved in Sent Items. This setting is disabled, by default. Keep it disabled to avoid the issue. The setting is found in File > Options > Mail > Save messages.
Ok so if you change this setting so that it does save copies, it actually doesn't duplicate the messages. Maybe it did in the past or for a user or two, but we've tested it and it doesn't. However when you sync, there is something somewhere that overwrites the manual selection of this option and keeps disabling the option. We have checked the clients registry and can see the SaveSent option change during the sync, even if we change it there, it still overwrites during the next sync.
What I want to do it remove this automatically disabling process when the sync occurs. Does anyone know how I can do this?