This error is valid if the user was not assigned the task or if the task had been completed.

1.  Is there any Outlook OOF rules that may be forwarding this email to the additional user?

2.  Is K2 OOF configured, as this would delegate the item to the user specified and may also send out an email.

3.  The original assignee to the task can also Delegate the task to this additional user (this will send out the email), but then completes the task before the delegatee can open the task via the email; if the task had already been completed; this error would also be thrown.

4.  Does the task notification have have CC: or BCC: configured, as this would send the email, but if the same user is not part of the destination set; then the task cannot be opened.

5.  If K2 Workspace is installed, how does this process instance look in the management worklist:

http://help.k2.com/onlinehelp/k2blackpearl/UserGuide/4.7/default.htm#6.Administer/Workspace_Legacy/Management_Console/Worklist.htm

Thank you for your response,

Actually i am not getting any email for this particuler activity because email is sending to a Dynamic UserID, 

Only  task is showing showing in my worklist and if i try to click on it its give an error as i mention above.

I have check everything for this activity but there is no any rule for destination user.

I have also removed this activity and re-designed it but the problem is same...