I had the "signatureField" bound to the "uid" in the solrconfig.xml, so the uid was always the same. Now I defined a new field for the "signatureField" and it works!
Before:
...
<updateRequestProcessorChain name="deduplication">
<processor
class="org.apache.solr.update.processor.SignatureUpdateProcessorFactory">
<bool name="overwriteDupes">false</bool>
<str name="signatureField">uid</str> <---------------------
<bool name="enabled">true</bool>
<str name="fields">content</str>
<str name="minTokenLen">10</str>
<str name="quantRate">.2</str>
<str name="signatureClass">solr.update.processor.TextProfileSignature</str>
</processor>
<processor class="solr.LogUpdateProcessorFactory" />
<processor class="solr.RunUpdateProcessorFactory" />
</updateRequestProcessorChain>
...
...
<fields>
<field name="uid" type="string" indexed="true" stored="true" multiValued="false" />
<dynamicField name="ignored_*" type="ignored" multiValued="true" indexed="false" stored="fasle" />
<field name="id" type="string" indexed="true" stored="true" multiValued="false" />
<field name="fullText" indexed="true" type="text" multiValued="true" />
</fields>
<uniqueKey>uid</uniqueKey>
...
After:
...
<updateRequestProcessorChain name="deduplication">
<processor
class="org.apache.solr.update.processor.SignatureUpdateProcessorFactory">
<bool name="overwriteDupes">false</bool>
<str name="signatureField">signatureField</str> <---------------------
<bool name="enabled">true</bool>
<str name="fields">content</str>
<str name="minTokenLen">10</str>
<str name="quantRate">.2</str>
<str name="signatureClass">solr.update.processor.TextProfileSignature</str>
</processor>
<processor class="solr.LogUpdateProcessorFactory" />
<processor class="solr.RunUpdateProcessorFactory" />
</updateRequestProcessorChain>
...
...
<fields>
<field name="uid" type="string" indexed="true" stored="true" multiValued="false" />
<field name="signatureField" type="string" indexed="true" stored="true" multiValued="false" /> <----------------------------------
<dynamicField name="ignored_*" type="ignored" multiValued="true" indexed="false" stored="fasle" />
<field name="id" type="string" indexed="true" stored="true" multiValued="false" />
<field name="fullText" indexed="true" type="text" multiValued="true" />
</fields>
<uniqueKey>uid</uniqueKey>
...