I’m not sure I understand, but… a question. Are you using a custom form or a native Glide form?
If you’re using native forms there is no way to prevent duplicate entries. This is because nothing is written to the table until the form is submitted, and so there is no way to detect the duplicate.
This is where custom forms come in. With a custom form, you target your input components at User Specific Columns, and the values are written to those columns immediately. You can then use those to detect duplicates before the form is submitted and take appropriate action.
Here is a concept app that demonstrates how to do it: