This code, using the docx library, accomplishes it:
private void RemoveSuperfluousSpaces(string filename)
{
bool superfluousSpacesFound = true;
using (DocX document = DocX.Load(filename))
{
List<int> multipleSpacesLocs;
while (superfluousSpacesFound)
{
document.ReplaceText(" ", " ");
multipleSpacesLocs = document.FindAll(" ");
superfluousSpacesFound = multipleSpacesLocs.Count > 0;
}
document.Save();
}
}
Download docx from here.