SOAP: can't create a lead with a duplicate email address in a different workspace

My instance has a Default partition, and an ABC partition.

The Default partition contains a Lead with email "batman@gotham.com".  This Lead has no foreignSysPersonID.  Last Name = Keaton.

I want to create a new Lead with the following properties:

Since Leads do not have to have unique email addresses, I think this should be possible.  You can certainly do it with the online portal.

I  used the following SOAP (some irrelevant details excluded)

<SOAP-ENV:Envelope blah blah>
  <SOAP-ENV:Header>
    <ns1:AuthenticationHeader>blah</ns1:AuthenticationHeader>
    <ns1:MktowsContextHeader>
      <targetWorkspace>ABC</targetWorkspace>
    </ns1:MktowsContextHeader>
  </SOAP-ENV:Header>
  <SOAP-ENV:Body>
     <tns:paramsSyncLead>

      <leadRecord>
        <foreignSysPersonId>1000</foreignSysPersonId>
        <Email>batman@gotham.com</Email>
        <leadAttributeList>
          <attribute>
            <attrName>LastName</attrName>
            <attrType>string</attrType>
            <attrValue>Kilmer</attrValue>
          </attribute>
        </leadAttributeList>
      </leadRecord>
      <returnLead>true</returnLead>
    </tns:paramsSyncLead>
  </SOAP-ENV:Body>
</SOAP-ENV:Envelope>

Instead of creating a new Lead in the ABC parition, it modifies the existing lead, changing Keaton to Kilmer, and presumably assiging the foreignSysPersonID to it too.

Even though I specified a foreignSysPersonId, it matched by Email anyway.

I even tried moving the email address into the leadAttributeList.  Same result.

What I'm trying to do is impossible via the SOAP interface.  Even though email addresses aren't unique, Marketo stubbornly wants to look stuff up by email addresses anyway.  (See also: your Munchkin library, which also subbornly uses email addresses as if they were unique).