Bibliography was duplicated when using "Categorize Categories"

Dear sir/madam,

I have a student who used “Categorize Categories” option in the EndNote tab in Word and now her Bibliography has duplicates citations. The section above has the original references and the “Uncategoried categories” section has the exact same references with some new ones.

When the “Group References by Custom Categories” was unchecked or deselected. This two portions remained with both portions have almost the exact same references.

The thing is how do I advise the student to remove the duplicated references? She want to choose one section and “highlight” and press the delete button for the other section, but I am worried this will mess up the documents and the in-text citations.

How do I advise my student.

Thanks in Advance,