When using Foxit Quick PDF Library it is possible to create checkbox form fields which have the same name and as a result become duplicate form fields (updating one updates them all), but it is not possible to have duplicate form fields with different export values.
When a duplicate form field is created what actually happens is that a parent form field is created which has two kid form fields. These kid form fields are linked via the parent form field. The two kid form fields are visible, but the parent form field is not visible.
Note: if you want duplicate form field behavior then the form fields must share the same name and same export value. If the names are the same but the export values are different then the checkbox form fields will behave as radiobuttons (selecting one checkbox will deselect the previously selected checkbox) and indeed the GetFormFieldType function will identify the form fields as radiobutton form fields.