How does Insights report on merged tickets?
A merged ticket is treated like any other ticket. The only thing that reliably sets them apart is that merged tickets have a closed_by_merge tag.
This means that metrics such as Full Resolution Time will not be combined with the ticket that it's been merged with; the two tickets are treated as completely separate tickets within Insights.
If you are looking to report strictly on tickets that have been closed by merge, you can do so by reporting on the closed_by_merge tag. For more information on tag reporting, see Insights tag reporting: Reporting on tickets with one or more tags.