Normally when we import records which has a related entity, import process will resolve the lookup reference if it finds single record in the parent entity otherwise, it will stop processing the record and ends in fail.
Ex: When we are importing a Contact (Contact1) with Parent Account (Account1) and system has two accounts with same name then Import process gives error when processing Contact1
Click on Edit in the mapping dialog
Go to Parent Account field in the list of mappings and click on Lookup
Set the unique field as the reference
Now in the import file add the data as Account Number for Parent Account field.
Hope this helps 🙂