I had the same problem and I solved it by adding Resource Column to the imported table, and checked the option "create if not existed" while importing. it wasn't the resource record duplicated, actually the record doesn't exist and that's the problem.
Just sharing it for the people who may face the same problem.
Hi. I did the same as you and it works fine. There´s a little detail though. When I imported employees I assigned the same name to the resource as to the employee which is the default behavior. The problem came when I realized that having thousands of employees, more than one employee had their names repeated. In this case I ended up with more than one employee pointing to the same resource since odoo assigned the employee to the existing resource. I ended up making the employee name unique to solve this. Cheers
Seems that importing data is referring to a record in the model resource.resource and that record with the given ID is not existing in the database. So Check the database and see whether there is a record present in the given ID.
Activate the developer mode and navigate to Settings -> Technical -> Resource -> Resources and check for the record existence.
For getting the sample import template of the Employees in odoo12,