The entity is already booked Error when trying to update a candidate record

Content

Hi -

I'm trying to update candidate record(s) and during my testing sometimes the candidate record will be updated and other times, I'll get "The Entity is already booked." error.  Any thoughts as to why this is occurring?  

Here is an example of my XML request (which is only updating two UDF fields):

<ns1:Envelope xmlns:ns1="http://schemas.xmlsoap.org/soap/envelope/">

   <ns1:Body>

      <ns2:update xmlns:ns2="http://www.taleo.com/ws/tee800/2009/01/candidate">

         <ns2:candidate xmlns:ns3="http://www.taleo.com/ws/tee800/2009/01/import">

            <ns3:Number searchType="search" searchValue="134455"></ns3:Number>

            <ns2:WorkdayApplicantID>A4014623</ns2:WorkdayApplicantID>