Fix duplicate parameters (in some cases)

This will avoid to add the same parameter names if they are declared so: this will happen sometimes when "instance-parameter" and "self" are declared in the same "parameters" element, such as `Gst-0.10.gir`:

      <instance-parameter name="object" transfer-ownership="none">
        <type name="Object" c:type="GstObject*"/>
      </instance-parameter>
      <parameter name="self" transfer-ownership="none">
        <doc xml:space="preserve">The XML node to load @object from</doc>
        <type name="libxml2.NodePtr" c:type="xmlNodePtr"/>
      </parameter>