You migrate one or more teams and get one of the following warning messages:
A team with the same name already exists in the destination. Your team will be copied as new and not merged with the existing team. We recommend renaming your team before copying.
When you migrate a team that already exists or you if you have a team with the same name in the destination, the app will create a new copy of your source team in the destination.
The new team in the destination will have the same name as the existing one. The site collection associated with your new team will have a number increment added to its URL.
We recommend that you change the name of your team in the Copy teams options if a team with the same name already exists in the destination. It will become increasingly difficult for your users to navigate your organization's Microsoft Teams if some of your teams have the same name.
Note: Though it is not possible to merge teams directly, you can copy the channels of a team into another team.