can't open the same buffer in a second frame

[ken]

It used to be nice that I could open the same buffer in a second frame 
(aka window).  Sometimes I need to edit the same file in three or four 
places and it's necessary to have all of them showing at the same 
time... and for other reasons.  But I tried doing "C-x 5 b" and also 
"C-x 5 f" yesterday and couldn't do it either way.  I could open a 
different (not currently open file or buffer), but couldn't open the 
same one.  It worked years ago. Why take away perfectly good and useful 
functionality?  More importantly, how do I get it back?