In article ,
nestork wrote:

gfre
Modern radar is sophisticated and might be able to tell the difference
between one plane or two located very close together. You'd need an
expert, like an air traffic controller, to provide an educated guess
here.

From talking with a friend who is an ATC about this in another
context said that would be very dependent on the strength and type of
radar, how close it is, and other obvious factors (not the least of
which is the attentiveness/skill of the ATC). So, especially at the
outer ends of the range, it could happen.
--
³Statistics are like bikinis. What they reveal is suggestive,
but what they conceal is vital.²
‹ Aaron Levenstein