Cool. Just to be perfectly clear, none of these sites are repeaters in the sense we think them for police radios and so forth. They're all discrete frequencies and it's the voice switch equipment used by ATC that allows them to cross-couple frequencies together. You won't see the same frequency used by the same ATC facility (for en route traffic) at multiple locations except in some very specialized situations. We do this in the northern part of Canada where we set up a "ring" of sites that all use the same frequency (for the same sector) and employ a voting receiver system to determine which site is used to transmit to and receive from any particular aircraft. The aircraft can all hear each other because of altitude, but the geographic area needing coverage means that ground based equipment cannot provide coverage from a single site. And we're talking the geographic area equivalent to several U.S. states combined, not a small pocket of territory. (Around Hudson Bay to be specific.)
If you hear the same aircraft on two or more frequencies simultaneously then for sure ATC is cross-coupling frequencies to absorb more airspace into the same sector during quieter hours. Any particular aircraft is only on one of the frequencies but it's being rebroadcast on the others so all the traffic hears everybody else.
BTW Manny, you should be able to find the physical location for each of the frequencies you monitor here (or elsewhere online) which should give you a rough idea of what area the sector is covering at any particular part of the day.
Here's a chart for Toronto ACC which gives each frequency, it's purpose and the location of the radio equipment. With Google Maps or similar at hand it's pretty easy to visualize the approximate coverage for each frequency. Hopefully you can find the same for your area.
Toronto (CZYZ) Scanner Frequencies and Radio Frequency Reference
Bob