Redmine

This plugin basically does what the name says: Merge duplicates.

The entire content of the source issue is copied into a new one. The attachments are shifted as well. The indikation of the merged content is made with some CSS around the duplicate text. As this is configurable you can give this the flavoured look you want. Just visit the configuration page of the plugin.

The duplicated issue is closed and cross linked to the new one, so you don't loose the reference. The state you want this ticket to be closed into can be chosen, as long as it closes it.
I registered it here: https://www.redmine.org/plugins/redmine_issue_merge

It is free for use, so you can git clone it, use it and further mess around with it.
Have fun