contributor appears multiple times with a user prefix when attaching files

1. on https://nightly.nuxeo.com/, login as Administrator
2. create a File document
3. click Add a File
4. save the document
5. export the document as XML
6. observe the following:

       <dc:contributors>
         <item><![CDATA[Administrator]]></item>
       </dc:contributors>
   

7. repeat the operation by replacing the file with another one
8. observe the XML export remains the same for contributors:

       <dc:contributors>
         <item><![CDATA[Administrator]]></item>
       </dc:contributors>
   

9. now add an attachment
10. observe the following XML export:

        <dc:contributors>
          <item><![CDATA[user:Administrator]]></item>
          <item><![CDATA[Administrator]]></item>
        </dc:contributors>
    

    The user Administrator appears more than once with a user prefix.

11. add 5 more attachments
12. observe the Administrator user appears now 7 times:

        <dc:contributors>
          <item><![CDATA[user:Administrator]]></item>
          <item><![CDATA[user:Administrator]]></item>
          <item><![CDATA[user:Administrator]]></item>
          <item><![CDATA[user:Administrator]]></item>
          <item><![CDATA[user:Administrator]]></item>
          <item><![CDATA[user:Administrator]]></item>
          <item><![CDATA[Administrator]]></item>
        </dc:contributors>
    

Expected result: the Administrator user should appear only once and without any prefix.