In our uCommerce site we only have one Shipping method, that is using a custom IShippingMethodService as we calculate pricing based on weight. The shipping service is correctly calculating the price, however for some reason the customer basket has 2 Shipment objects, with different ShipmentId values but the same ShippingMethod.ShippingMethodId values

This is my code, with some comments

TransactionLibrary.EditShippingInformation(
                stage1Model.PersonalDetails.FirstName, 
                stage1Model.PersonalDetails.Surname,
                stage1Model.PersonalDetails.EmailAddress, 
                stage1Model.PersonalDetails.TelephoneNumber, 
                stage1Model.PersonalDetails.TelephoneNumber,
                stage1Model.PersonalDetails.Company, 
                previousStageModel.DeliveryAddress.Address1, 
                previousStageModel.DeliveryAddress.Address2,
                previousStageModel.DeliveryAddress.Postcode, 
                previousStageModel.DeliveryAddress.City, 
                previousStageModel.DeliveryAddress.County,
                "", 
                countryId);

var currentShippingMethod = TransactionLibrary.GetShippingMethod();

//The currentShippingMethod.ShippingMethodId is 9

TransactionLibrary.CreateShipment(currentShippingMethod.ShippingMethodId, overwriteExisting: true);

//At this point the basket has 2 Shipment objects, even though I'm passing the overwriteExisting = true;

TransactionLibrary.ExecuteBasketPipeline();