Issue while modifying and importing the manually content type data

I have a scenario like this:

I am using Feeds Importer to import one content type data. I am using GUID field as the unique field. Also, I am exporting these all the nodes of this content type using Views into a .csv file.

My problem is that users can create nodes using the Feeds Importer and also manually in the Drupal admin console.

While I export the nodes of the content type, it contains all nodes (manually created nodes and Feeds imported nodes). There will be no GUID for the manually created nodes.

In this case, how can I resolve the issue of re-import of this data into the Drupal tool?

Is there a way to update the GUID field in the database whenever a new node of this type is created? This will resolve the issue.