Rest call to delete shipping lines

Sometimes, when using REST and trying to delete all the lines on a packslip.
This is the rest service I’m hitting from a loop is: https://my-server/E10Live/api/v1/Erp.BO.CustShipSvc/ShipDtls(SD,123456,1)
I hit the endpoint in succession until all the packlines I want are deleted. Sometimes I get this message and the line isn’t deleted:

The table ttShipDtl has more than one record

What is the business object trying to telling me? Why can’t I delete more than one record? Why is there more than one record to begin with?