GSMMO best practices
When importing data with G Suite Migration for Microsoft® Outlook®...
If you're importing a lot of data and don't want to tie up your computer, pause the migration while you're working and resume when you step away from your desk (such as when you go to lunch). After resuming, the migration continues where it last left off.
You might also want to avoid migrating during business hours but instead do so at night or over the weekend. If your mailbox gets too busy, some messages might be delayed.
Avoid re-importing contact data that you've already migrated, or you'll end up with duplicate contacts. If you need to run another migration and have already imported your contacts, either uncheck Contacts in the migration wizard, or choose the option to Migrate only new data.
Avoid re-importing calendar data if you've changed any events in Google Calendar since you last migrated. Importing calendar data overwrites existing events in Google Calendar so you'd lose your recent changes.
If you need to re-import contact and calendar data, such as after performing a test migration, delete the data your originally migrated, first. Then run a new migration.
Don't run two migrations at once, for example, to import data for more than one G Suite account. If you're performing migrations for multiple users, import only one account at a time.