I have seen (heard) this as well, but not for a long time!
In QLD, I have heard Dubbo Flight Service and Port Morseby. I cannot recall the details though.
In the 70's, a lot of places had the same frequencies. eg., most Towers were 118.1, and the equivalent Control freq was 118.7, so it wasn't unusual to hear the adjacent TWR on rare occasions!
Got no idea of the physics behind it?