Merge issue feature needed

We often get multiple requests for the same thing. Sometimes it is because of human error and other times because of a customer replying to a notification via a different email address. 

We MUST have the ability to be able to merge issues. This is standard in all the leading service and support desk systems. Our customers (internal) all log in via single sign on. However half the users use an alias as their primary email and this means we get duplicate issues for 50% of requests as soon as the customer replies to a question. 

Currently we have to delete the new "duplicate" as the SLA would be inaccurate and the user would get very confused.