Removed redundant check in scope constructing

This actually never happens due to the nature of indices. Each left
bracket with a given index is uniquely associated with its matching
right bracket with the same index: it would be the closest one. Also,
there can be no other bracket pairs with the same index between two
matching brackets. There is no way we can mistake one right bracket
for another and incidentally mess up pairs. Thus the check can be
omitted.