Symptoms

I have a process where an approver can approve or decline a process. When the user actions the task item with Smartactions, they receive two replies.

When the Approver replies with --> Approve, he receives two replies:
You successfully completed the Review task with the 'Approve' action.

and

The K2 server could not find the worklist item mSN]. This item may have been actioned by another user.

Response "2." is unnecessary and confuses the approver. This does occur 100% of the time. It is not a once in a while.
 

Diagnoses

This was related to having a Smartactions mailbox that was shared by both the Production and Development K2Servers.
 

Resolution

We needed to set up a new account for smartactions to take place, and then set the smartactions to it. Once the new account is set up and ready to go, simply got the K2HostServer.exe.config and search for "system self" This will bring you to a tag that looks like this:
<system self=" K2SmartactionMailboxAddress]" enableListeners="true" allowAmbiguity="true" />

Change the value of the oK2SmartactionMailboxAddress] to use the new account. Once this is done, restart the K2Service.
Also, make sure that your EWS Connection connection string is pointing to the correct server where you created the new Smartactions account.