A more thorough explanation:

Duplicate-claiming rejection is a type of rejection in patent law where a patent claim is rejected because it is not substantially different from another claim. This means that the same invention is being claimed twice, which is not allowed.

For example, if someone tries to patent a new type of phone case and makes two claims: one for a blue phone case and another for a red phone case, both claims may be rejected if they are not substantially different from each other.

This type of rejection is meant to prevent people from monopolizing an invention by making multiple claims for the same thing.