When concrurrent processes make use of eZContentFunctions::updateAndPublishObject() to update the same object, it results in "Duplicate entry" errors.

It seems to me, the problem comes from the eZContentObject::nextVersion() method :

function nextVersion()
{
    $db = eZDB::instance();
    $versions = $db->arrayQuery( "SELECT ( MAX( version ) + 1 ) AS next_id FROM ezcontentobject_version  WHERE contentobject_id='$this->ID'" );
    return $versions[0]["next_id"];
}

This SQL request returns the same next version number in several concurrent processes. This is normal from a transactional database point of view.

To avoid this problem, we should use "SELECT ... FOR UPDATE" (cf https://dev.mysql.com/doc/refman/5.0/en/innodb-locking-reads.html).
I checked and eZContentObject::nextVersion() is only called in eZContentObject::createNewVersion(). So using "SELECT ... FOR UPDATE" makes sense.

I tried but I experimented SQL deadlocks.
It becomes complicated to debug and fix the problem as there are many complex nested transactions in the corresponding eZ Publish Legacy code.

So I'm sorry I don't provide any fix yet.