File to RFC to File - MSGGUID_EXISTING Error

I have implemented File to RFC to File scenario. I had it working just fine but have been posed by the following error on the Sender file adapter

Error: com.sap.engine.interfaces.messaging.api.exception.MessagingException: XIServer:MSGGUID_EXISTING:

I have tried the following

1. Start/stop channel

2. Restart XI server - In the past we had this problem and restarting the XI server did the trick.. but not working this time around.

Thanks, any help is appreciated