cell-renderings.lua silently drops duplicated render names

[gordonwoodhull]

As for why it validates with four plots - there are other uses for this such as showing different versions of a plot on mobile vs desktop, so it's more general than just light and dark. It should probably warn if you reuse light and dark - the extras are discarded.

Originally posted by @gordonwoodhull in #13893 (comment)