I have not seen it noted that when Washington Center RCAG site at Bucks Elbow combines the Montebello (121.675) and Azalea Sectors (135.4) when traffic is light enough to allow, each frequency acts as a repeater repeating not only Washington Center *but also the aircraft that was on the other frequency*. So if you are within range of hearing Bucks Elbow and you hear an aircraft on 121.675 or 135.4 at these times, it may actually be transmitting on the other frequency. (If you listen to both, you can usually determine which frequency the aircraft transmitted on as that will generally be stronger.) Essentially each frequency acts as an input to the other; of course some logic circuitry is necessary to avoid a "feedback effect". The UHF frequencies (263.1 and 284.7, respectively) are also simultaneously transmitted.