[NTG-context] small indexing glitch if index entry has extra {}

Here's a small glitch with indexing, which I ran into by chance and enclose a minimal example. Because of the extra {} in the entry \index{{two dimensional}} the index duplicates the entry: t two dimensional 1 two dimensional 1 \setupregister[index][n=1] \starttext \index{two dimensional}test1 test \index{{two dimensional}}test2 \blank[3*big] \placeindex[criterium=all] \stoptext