fix for issue duplicate keys requested in vizier
@@ -536,6 +536,9 @@ class VizierClass(BaseQuery):
if columns is not None:
columns = self.columns + columns
+ # filter columns to _unique_ columns, preserving order in python >3.6
+ # note that "set" does not preserve order, but dict.keys does
+ columns = list(dict(columns).keys())
# special keywords need to be treated separately
# keyword names that can mean 'all'
