isentropic_interpolation_as_dataset returns unplottable dataset about metpy

[dopplershift]

Interesting, so xarray only complains about the duplicates when you try to select along the axis with duplicated values. 🤔

Changed to enhancement because MetPy isn't doing anything wrong, it did exactly what was asked and produced correct values for that case--even if the net behavior is sub-optimal. I do think it would be nice if we can find a clean, sensible solution to address it, though there's a limit to how much I'm willing to patch around bizarre corner cases when individual pieces are operating themselves completely sensibly.