title: Solr field UUID is not unique
body: <p>I'm running Solr 5.1.0 on our Plone 4.2.6 system. In my <code>schema.xml</code> I set UID to be the uniqueKey. On the system there are approx 82.000 documents. After building an Index, I found the following amount of indexed docs:
  <code>
  numDocs: 54537
  maxDocs: 82561
  deletedDocs: 28024
  </code>
  Furthermore under 'core-specific-tools > Schema Browser > Load Term Info > Histogram' I found that for the field UID there are 26,514 values of UID contained in only one document but also 28,022 values contained in 2 documents. Therefore that amount of indexed documents were tagged as deletable.</p>
  
  <p>Can anyone tell me what reason there could be for that many UIDs to be the same although they should be distinct?</p>