Libreplan XML import issue

I try to import some XML file into Libreplan REST API and get the below error message:

<?xml version="1.0" encoding="utf-8"?>
<instance-constraint-violations-list xmlns="http://ift.tt/1Cx6EPx">
  <instance-constraint-violations entity-type="order-element" code="ORDER0005" num-item="1">
    <constraint-violation message="Duplicate progress assignment for task ORDER0001-0005" />
  </instance-constraint-violations>
  <instance-constraint-violations entity-type="order-element" code="ORDER0008" num-item="2">
    <constraint-violation message="Duplicate progress assignment for task ORDER0008" />
  </instance-constraint-violations>
  <instance-constraint-violations entity-type="order-element" code="TEST0001" num-item="7">
    <constraint-violation message="Duplicate progress assignment for task TEST0001" />
  </instance-constraint-violations>
  <instance-constraint-violations entity-type="order-element" code="TEST0002" num-item="8">
    <constraint-violation message="Duplicate progress assignment for task TEST0002" />
  </instance-constraint-violations>
</instance-constraint-violations-list>

The file is actually the same as the XML which was exported by the REST API for orderelements. Based on the error message I suspect Libreplan tries to insert the entities into its database as new, however its supposed the update them.

Can anyone help? Is there any documentation where I could get any leads? I haven't happened to find any unfortunately.