The duplicate supplier dialog will appear when you are importing a supplier from another Resort Software database and that supplier already exists in the destination database. See the following image.
To keep the existing data in the destination database, click the "Keep existing" button and the supplier you are importing will be discarded.
To overwrite the existing supplier with the supplier you are importing, click the "Overwrite existing" button.
To cancel the entire import operation, click the "Cancel operation" button and you will be returned to the last opened form.