Do not Reset the current position of migration to start from the beginning
In the Destination database, the data migration tool will truncate the table but we need the data that exist in the destination database.
Is there anyways run the data migration tool without touch previous data in the destination database.
The '-r' switch will reset your data transfer to the start
bin/magento migrate:data -a -r vendor/magento/data-migration-tool/etc/opensource-to-opensource/1.9.3.9/config.xml
If you continue to have issues you can drop and recreate your target database on Magento 2 and reinstall Magento. (It is easier to take a snapshot of your database when it is clean so it is easy to reset if needed.)
drop database <db_name>;
create database <db_name>;
This will ensure you don't have a database that is ready to migrate to.
