Another "coalesce=False" join schema issue

[alexander-beedie]

@nameexhaustion: Up for another one? 😉

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

import polars as pl

lf1 = pl.LazyFrame({
    "a": [1, 2, 3, 4, 5],
    "b": [1, 2, 3, 4, 5],
})
lf2 = pl.LazyFrame({
    "a": [0, 2, 3, 4, 5],
    "b": [1, 2, 3, 5, 6],
    "c": [7, 5, 3, 5, 7],
})

lf3 = lf1.join(
    other = lf2,
    how = "inner",
    left_on = ["a", "a"],  # << note: joining on `a = b` *and* `a = c`
    right_on = ["b", "c"],
    coalesce = False,
)

lf3.collect()
# shape: (2, 5)
# ┌─────┬─────┬─────────┬─────────┬─────┐
# │ a   ┆ b   ┆ a_right ┆ b_right ┆ c   │
# │ --- ┆ --- ┆ ---     ┆ ---     ┆ --- │
# │ i64 ┆ i64 ┆ i64     ┆ i64     ┆ i64 │
# ╞═════╪═════╪═════════╪═════════╪═════╡
# │ 3   ┆ 3   ┆ 3       ┆ 3       ┆ 3   │
# │ 5   ┆ 5   ┆ 4       ┆ 5       ┆ 5   │
# └─────┴─────┴─────────┴─────────┴─────┘

Now select the columns "a", "b", and "c" from that same LazyFrame:

lf3.select("a", "b", "c").collect()
# DuplicateError: the name: 'a' is duplicate

Issue description

Selecting columns from a post-join LazyFrame can raise an erroneous DuplicateError.

Expected behavior

The above join + select operation should result in the following frame:

# shape: (2, 3)
# ┌─────┬─────┬─────┐
# │ a   ┆ b   ┆ c   │
# │ --- ┆ --- ┆ --- │
# │ i64 ┆ i64 ┆ i64 │
# ╞═════╪═════╪═════╡
# │ 3   ┆ 3   ┆ 3   │
# │ 5   ┆ 5   ┆ 5   │
# └─────┴─────┴─────┘

Installed versions

Compiled current main (as of a few minutes ago) with all standard features.

[ritchie46]

@nameexhaustion can you take a look?

[nameexhaustion]

Will take a look