Mongo Storage Engine from mmap to wiredTiger
That error implies there is already data in TestDataBase.testCollections. You might consider using the --drop option:

Before restoring the collections from the dumped backup, drops the collections from the target database. --drop does not drop collections that are not in the backup.
When the restore includes the admin database, mongorestore with --drop removes all user credentials and replaces them with the users defined in the dump file. Therefore, in systems with authorization enabled, mongorestore must be able to authenticate to an existing user and to a user defined in the dump file. If mongorestore can’t authenticate to a user defined in the dump file, the restoration process will fail, leaving an empty database.