I don't think there's a limit on how often you can transit a city but you can only fly one RT between two cities. For example if you fly AUS-DFW-ORD-DFW- AUS, you can't fly AUS-DFW or DFW-ORD either way again.

I had to change my above itinerary, and it's now out for pricing. Ayers Rock just wasn't going to work. Later found out it might have, but I like the new one just as well: hnd-jfk-dfw-ict-dca-ict-dfw-dub-doh-agp-hel-doh-akl-mel-cns-syd-hnd. 45396 actual flown miles according to GCM.
After taking out AYO I had a segment to add somewhere and picked Europe. There was only one segment available so I couldn't connect through DOH again. AGP-HEL-DOH is not on QR (too bad), but it is on AY, another 150% D fare bonus!
The reason they couldn't find availability from AYO-SYD came to light when the agent was told we were up against the 331 day out reservation limit. Although she saw it all as available when I was giving her my flights, when she tried to book it they finally, after about 20 minutes, realized the problem. So I have to call back Jan 20 to fix the dummy dates for my last two flights.