Unfortunately, I'm Mathcadless at the moment, but if you look in the worksheet I've attached to this message http://communities.ptc.com/message/266834#266834 I hope you will find the functions set and setcount in the collapsed Utilities Area. set (stack (A, B)) should do the trick. set sorts, though ... I do have a version that doesn't sort somewhere that checks if an element is in the 'set' vector (initialized to the first element of the input vector) and adds the element of it isn't., but I don't know where it is.
Stuart