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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_files_for_document(document):
 """ Returns the available files for all languages. In case the file is already present in another language, it does not re-add it again. """

    files = []
    for doc_trans in document.translations.all():
        if doc_trans.filer_file is not None and \
                doc_trans.filer_file not in files:
            doc_trans.filer_file.language = doc_trans.language_code
            files.append(doc_trans.filer_file)
    return files