the "dup" or "duplex" button is for the "range extender" feature of your radio.
when transmitting on a channel between 1-8 in duplex mode, your radio is actually transmitting on 31-38. as soon as you let go of the PTT button the radio will go back to receiving on channels 1-8.
eg. using ch1 duplex will transmit on ch31 and then receive on ch1 again.
when your using a repeater, your signal will be received by the repeater (usually on top of a mountain somewhere that it has good line of sight over the surrounding areas) and then retransmitted with a better signal to give you more coverage.
basically if your on one side of a mountain and your mate is on the other, you wont be able to communicate directly so you use the repeater.
the problem is when someone uses say ch31 because they think it is a clear channel and cant hear anyone they are actually being retransmitted on ch1 (if there is a repeater that can receive the signal. and you would be supprised how far your signal will go. you may not be keying it up continually, but still enough to be irritating)
another example. on the weekend at nissan trials i noticed there were some people using ch33 and intermitantly keying up the ch3 repeater at appin.
and the suzuki club using ch1 simplex when the ch1 repeater could be clearly heard at the same time.
ch5/35 are also used for EMERGENCYS ONLY. and there are people that do moniter these channels.
CREST is one organisation that does.
as for 22/23 ausoops has explained that one.
ch40 is set aside as the "road channel" but the truck drivers have decided that one channel isnt enough so they use 29 also.
and also in sydney its a common problem when they start on 40, hear a mate and say "go down one mate" until they find a clear channel. most of the time ending up on 37 or 35
