matplotlib.figure.Figure.legend used to check if a line had the same label as an existing legend entry. If it also had the same line color or marker color legend didn't add a new entry for that line. However, the list of conditions was incomplete, didn't handle RGB tuples, didn't handle linewidths or linestyles etc.

This logic did not exist in .axes.Axes.legend. It was included (erroneously) in Matplotlib 2.1.1 when the legend argument parsing was unified [#9324](matplotlib#9324). This change removes that check in .axes.Axes.legend again to restore the old behavior.

This logic has also been dropped from .Figure.legend, where it was previously undocumented. Repeated lines with the same label will now each have an entry in the legend. If you do not want the duplicate entries, don't add a label to the line, or prepend the label with an underscore.