This function works similar to unique, but provides additional information about which elements of original input 'x' are repeatd by providing indexes realtoe to the input.
firstOfRepeated makes list with 3 elements : $indRepeated.. index for first of repeated 'x', $indUniq.. index of all unique + first of repeated, $indRedund.. index of all redundant entries, ie non-unique (wo 1st).
Used for reducing data to non-redundant status, however, for large numeric input the function nonAmbiguousNum() may perform better/faster.
NAs won't be considered (NAs do not appear in reported index of results), see also firstOfRepLines() .