Integrity constraint violation: 1062 Duplicate entry '0' for key 'PRIMARY'

<ignore>
    <document>customer_group</document>
 </ignore>

worked for me inside the map.xml (or map.xml.dist) source -> field_rules node

The problem should go away if you change bulk_size to something other than 0 in your config.xml. It doesn't seem to auto-detect correctly on the customer_group table. Setting mine to 100 solved the problem for me.

To find out what table is the problem, try adding some debugging info to your src/Migration/Step/Map/Data.php file. Something like $this->logger->info("DEBUG: " . $sourceDocName ); around line 136. This should output the table the migration tool is currently working on in the console.

I had the same problem.. to get past it at least temporarily, I added this to the map.xml file under source and under document_rules

 <ignore>
    <document>customer_group</document>
 </ignore>

I am getting the same error. When doing the migration with verbose mode on, I can see that the issue is with the "customer_group" table. Primary key 0 is the "NOT LOGGED IN" customer group.

That is how far I am into for now. I will let you know if I find anything else.