Specified Visual is already a child of another Visual or the root of a CompositionTarget.

This problem will raise because of you are using the same viewport twice or more than once. if the situation is like that first you should clear the viewport by calling the method "clearViewport()",what you are loading then check whether it is null or not.
if the viewport is null then add the childrens to that viewport else return.
this is the method for clearing the viewport.
private void ClearViewPort()
{
ModelVisual3D viewPortChildLocal;
for (int i = viewport.Children.Count - 1; i >= 0; i--)
{
viewPortChildLocal = (ModelVisual3D)viewport.Children[i];
if (viewPortChildLocal.Content is DirectionalLight == false)
{
viewport.Children.RemoveAt(i);
}
for checking the viewport if it is null or not

ModelVisual3D visual = new ModelVisual3D();

ClearViewPort(); // first clear
if (visual != null) // and need to check if it is null or not
{
viewport.Children.Add(visual);
}
else
{
return;
}