email integration

I know that end user can perform actions throw email reply like add new comment and So on.
And when end user tries to perform an action (like add new comment) for the second time (replying to the same email more than one time to add new comments) this action fails because this  action has already been performed .
My question is :  How can end user add comments or status updates to record via email more than one time.
For example : an end user has submitted a request and after one hour without any updates he want to send an e-mail to be a comment (or statues update) in the discussion without logging in service portal.
how can I perform that?
I wish my question is clear and obvious.