Well, the software that MOST use in programming the GRE (Radio Shack 106 and 197) Scanners allows you to check for Duplicate frequencies. And you can set Dupe in your GRE scanner and it will tell you the freq already programed.
Having said that there are times when you need a dupe frequency. Example in conv. freq. most fire depts around here use 154.370 for tone out, if you don't set a squelch tone for that freq you will hear every fire dept within a hundred miles. So in my case there are two county dispatches that I want to hear on that freq, I have a dupe with different squelch tones for that freq. If you run a dupe check with the software they show up as dupes.
The same can apply for MARC TG that are borderline on one tower or another while your mobile setting a dupe assigned to a different tower works good for that too.
Rick