Hi Dave,

Simplest solution to this is to download this file:

http://www.projectpluto.com/comets.dat

(42473 bytes) and use it to replace the existing
'comets.dat' in your Guide folder.

Most people won't run into this issue. But if you've
used the 'Add MPC Comets/Asteroids' option to add asteroids,
then the headache in question will most definitely arise.
You may have downloaded an object known as, say, 2003 XY123,
long ago; since then, it's become numbered object (123456)
Randomname, listed under that name in the latest 'mpcorb.dat'.
Guide has no way to realize that these objects are really one
and the same, so it plots them both.

The above 'comets.dat' contains only comets, with no
asteroids at all, so no chance of doppelgangers.

Even more people will run into a related issue: comets are
often found under one designation, then given a permanent
periodic designation. For example, P/1997 V1 (Larsen) just
got the designation Larsen (200P). Unless you go in by hand
to remove the first one (or download the above file, in which
all such cases are corrected), Guide will show them both.

It occurs to me that there's a solution to the asteroid
problem: if 'mpcorb.dat' is in use, then MPC comets from
'comets.dat' should be suppressed. (Guide can tell which
objects came from MPC and which didn't.) The comet problem
is a much hairier one. Ideally, MPC (or someone) would
provide a comprehensive comet file, much as it does for
asteroids. But it doesn't look as if that's in the works.

-- Bill