You need to pop off the label from kwargs first in order to prevent duplicate kwarg entries.

That is done way at the top, seehttps://github.com/matplotlib/matplotlib/blob/master/lib/matplotlib/axes/_axes.py#L2789

The two major over-hauls that need to happen in errorbar are a) use z-order to specify above/below so we only have one code path there b) promote containers to full artists so that theLine2D objects don't end up in thelines attribute and effectively in the legend (but not draw!) search path twice.

Ah, ok. This makes sense to me as a good stop-gap solution.