double file entry passed

I have made both file fields required, but if you submit exactly the same file in both fields, validation passes it, and it emails only one copy of the tandem files.
http://enlaceslanguagesolutions.com/index.php/en/document-translations

Well yes. Both files are required, and both have a file selected. You need to check for DUPLICATE files, which you'd do in the submit event. You'll need to do it with PHP and there are a range of options on Google for it.