Just a quick question, if I'm understandig it correctly: The Redeem RSL Promo Codes work only, if you haven't ever used the code before?

I used the button "redeem now" and tried a few codes, which were suggested on the internet, like Demolish or DragonEgg. When I pressed "confirm" it says the code is invalid. With Demolish and DragonEgI know for sure, that I used them, when they came out, so invalid might be correct. 

Would be grateful for some clarification!