Join errors when join keys have the same name

[kevinzwang]

Describe the bug
When multiple join keys have the same name on the same side, the join errors with

DaftCoreException: DaftError::External Unable to create logical plan node.
Due to: DaftError::ValueError Attempting to make a Schema with duplicate field names: duplicated_name

To Reproduce
Steps to reproduce the behavior:

import daft
from daft import col

df1 = daft.from_pydict({
	"a": [1, 2],
	"b": [2, 2]
})

df2 = daft.from_pydict({
	"a": [1, 2],
})

# does not work
df1.join(df2, left_on=["a", "b"], right_on=["a", "a"])

# works
df1.join(df2, left_on=["a", "b"], right_on=["a", col("a").alias("not_a")])

Expected behavior
Both joins above should result in the dataframe without error.

[kevinzwang]

May be a good first issue for someone who wants to learn more about how we do joins.