[BUG] Redundant axes clause leads to error

[dorisjlee]

Describe the bug For bar charts, if there are more than two of the same attributes specified, then reset_index in execute_aggregate throws an error.

To Reproduce

df = pd.read_csv("https://github.com/lux-org/lux-datasets/tree/master/data/airbnb_nyc.csv?raw=True")
from lux.vis.Vis import Vis
vis = Vis([lux.Clause("room_type",channel="y"), "number_of_reviews",lux.Clause("room_type",channel="color")], df)