Deterministic content models???

I received the following error when I parse an xml document:

>> The content model must be deterministic. Multiple definition of
element 'DATA' causes content model to become ambiguous. 

Here is the element in question:

<!ELEMENT SMART_DOCUMENT (HEADER , (DATA , VIEW*) , AUDIT_TRAIL ,
SIGNATURES?)>
<!ELEMENT DATA (MAIN , MAP? , CUSTOM?)>

Can someone be kind enough to explain me why this is not a valid DTD? 

I think the concept is that there can be multiple DATA sections and
multiple VIEW sections within one document.


TIA,
Andrew Dubinsky

P.s. I did not create the DTD, so I do not have the ability to edit it.
I am not using the DATA element more than once in the document.