Fix index duplication

[dylanhmorris]

• Closes Dataset.to_dataframe() and DataArray.to_dataframe() convert multi-index levels both to pandas MultiIndex and to columns #10538
• Tests added that fail without the fix. Have confirmed that test suite passes locally with the fix.
• User visible changes (including notable bug fixes) are documented in whats-new.rst
  I am not sure this bug fix is notable enough to be documented in whats-new.rst, but I can add if maintainers disagree.

I would be grateful for confirmation from someone more experienced with the codebase that my reasoning here is correct:

xarray/xarray/core/dataset.py

Lines 7186 to 7188 in ff03b8a

	# All and only non-index arrays (whether data or coordinates) should
	# become columns in the output DataFrame. Excluding indexes rather
	# than dims handles the case of a MultiIndex along a single dimension.

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