Finding ALL duplicate rows, including “elements with smaller subscripts”

Question

R\'s duplicated returns a vector showing whether each element of a vector or data frame is a duplicate of an element with a smaller subscript. So if rows 3, 4,

Answer 1

I've had the same question, and if I'm not mistaken, this is also an answer.

vec[col %in% vec[duplicated(vec$col),]$col]

Dunno which one is faster, though, the dataset I'm currently using isn't big enough to make tests which produce significant time gaps.