When two tickets are merged, "merge this" ticket still keeps it's own SLA - Bug Report - Deskpro Support
When merging two tickets, the "merge this" ticket doesn't take on the primary tickets SLA and if an email alert is set for an SLA violation you receive an email for the "merge this" ticket.
When merging two tickets, the "merge this" ticket keeps it's SLA, so when you have emails set to send SLA violations an email still goes out even though the primary ticket's SLA is completed.
This is by design. If you don't want both SLAs on a ticket after merge, you would need to delete one.