How do I resolve an issue as a duplicate?

[noamdah]

Set the issue resolution to duplicate if you don't have that resolution you can add it to you jira.

[Nic Brough -Adaptavist-]

That's a matter of your process.  However, a simple, and commonly used option with JIRA is to simply close it and put the "duplicate" resolution on it while doing so.  More complex variations might include transitions for "resolve as a duplicate" that would set the resolution for you and probably prompt for a link to the original.