PDField.getFullyQualifiedName() returns name adding suffix '.null'

We have several pdf files where each one contains one pdf form with their own fields. We need to read all pdf fields and list them into a txt file.

The problem comes when a pdf form has duplicated field names, so the field.getFullyQualifiedName() returns the name of the field wrong, adding '.null' at the final of field's names.

-->Situation:
1) PDf file containing a pdf form
2) The pdf form contains lot of fields, some of their field's names are duplicated, like for example 'Applicant.city'.
3) When I try to list all of field's names, duplicate field's names comes with a suffix '.null' --> this only happends on duplicated field's names.
----------------------------------------------------------------------------------------------

-->Example:
1) PDF Form with 4 fields whos names are: 'Applicant.name', 'Applicant.phone', 'Applicant.ssn', 'Applicant.name'.

2)After running the code shown bellow, the result list is: 'Applicant.name.null', 'Applicant.phone', 'Applicant.ssn', 'Applicant.name.null'.
----------------------------------------------------------------------------------------------

-->Attach the code for listing all pdf form field's names:

public static Set<String> printFields( PDDocument doc ) throws IOException {
PDDocumentCatalog docCatalog = doc.getDocumentCatalog();
PDAcroForm acroForm = docCatalog.getAcroForm();
List fields = acroForm.getFields();
Iterator fieldsIter = fields.iterator();

Set<String> fieldSet = new HashSet<String>();

while ( fieldsIter.hasNext() )

{ PDField field = (PDField)fieldsIter.next(); // String fieldFullName = processField(field); fieldSet.addAll( processField( field ) ); }

return fieldSet;
}

private static Set<String> processField( PDField field ) throws IOException {
List kids = field.getKids();
Set<String> result = new HashSet<String>();
if( kids != null ){
Iterator kidsIter = kids.iterator();

while ( kidsIter.hasNext() ){
Object pdfObj = kidsIter.next();
if( pdfObj instanceof PDField )

{ PDField kid = (PDField)pdfObj; result.addAll( processField( kid ) ); }

}
}else

{ //System.out.println( "field.getFullyQualifiedName(): " + field.getFullyQualifiedName() ); result.add( field.getFullyQualifiedName() ); }

return result;

}
--------------------------------------------------------------------------------
field.getFullyQualifiedName() is returning duplicated field's names with a prefix '.null'.

Thanks in advance.