[nofish]

You could compare against previous state and only execute if state has changed compared to previous state, something like this:

prevState = -1

function main()
  A=reaper.JS_Mouse_GetState(-1)
  reaper.defer(main)
  if prevState ~= A then
   if A == 5 then
      reaper.ShowConsoleMsg("CMD+Click")
    end
  end
  prevState = A
end

main()

Makes use of a global variable though, not sure if there'd be a way around it..