XML Serializer C# Namespaces Duplicate Issue

I am using XmlSerializer in C# to generate an XML document based on a model. I need to generate the following XML root that contains a duplicate namespace using separate prefixes. Below is what the output should look like…

<ClinicalDocument xmlns:xsi="http://ift.tt/ra1lAU"
 xsi:schemaLocation="urn:hl7-org:v3 CDA_SDTC.xsd"
 xmlns="urn:hl7-org:v3"
 xmlns:cda="urn:hl7-org:v3"
 xmlns:sdtc="urn:hl7-org:sdtc">

However, when I Serialize this, the default entry is removed (which contains the duplicate namespace) and the root is prefixed.

<cda:ClinicalDocument xmlns:xsi="http://ift.tt/ra1lAU" xmlns:sdtc="urn:hl7-org:sdtc" xsi:schemaLocation="http://ift.tt/ra1lAU" xmlns:cda="urn:hl7-org:v3">

Here is my XmlSerializer code...

var writer = new XmlSerializer(clinicalDocument.GetType(),"urn:hl7-org:v3");

            var myNamespace = new XmlSerializerNamespaces();

            myNamespace.Add("sdtc", "urn:hl7-org:sdtc");
            myNamespace.Add("xsi", "http://ift.tt/ra1lAU");
            myNamespace.Add("cda", "urn:hl7-org:v3");


            using (var file = new System.IO.StreamWriter(CCDUncOutputPath))
            {
                writer.Serialize(file, clinicalDocument, myNamespace);
                file.Close();
            };

            writer = null; 

            GC.Collect();

Does anyone have a fix for this?