I think something like this should work. Try it out and let me know.
/castsequence reset=11 !Retribution Aura,Crusader Strike,Crusader Strike,Crusader Strike,Crusader Strike,Crusader Strike,Crusader Strike,Crusader Strike,Crusader Strike,Crusader Strike,Crusader Strike,Crusader Strike
It's basically your original macro but with an exclamation mark before the aura cast !Retribution Aura
When you put an exclamation mark before a spell in a macro it will only be cast if it's not already active. It helps with spells that toggle on/off. If it's off it will turn it on and if it's on it will leave it alone.
Edit: I tested the macro and it does work. I still don't really understand what you're trying to accomplish. I see a couple of issues with macroing Ret Aura with Crusader Strike.
One, it's a waste of a global cooldown even when it's a non-cast. If the aura is on, it won't toggle off but it will use up a GCD as it goes through the cast sequence. This means every time the macro resets and you walk up to a new mob the first thing you will do is stand there for 1.5 seconds waiting on GCD. On long bosses it's another DPS nerf since you will waste a GCD every 44 seconds or so.
Two, Ret Aura is pretty crappy. If I remember correctly it hits for 150-200 every few seconds? I don't think it's a great idea to assume it will always be your best choice and bind it to your DPS sequence. I usually run with Devo or Concentration in parties and raids unless I'm expecting a certain type of elemental damage.
Like Jendah said above, Pally Power is your friend for auras and buffs.