Thankful can automatically merge duplicate requests from the user if they write in multiple times about the same subject within a 3-day period using the option in the Dashboard’s Settings->Behavior page.
However if you want to control how and when merges happen, such as when you have multiple brands in the same helpdesk and want to ensure that tickets across brands from the same user are never merged together, this step gives you that level of control in triggers.
In the typical use-case of one-brand, one-helpdesk you should use the global setting instead.