Hi
In order to play SimCity: Cities of Tomorrow, you'll need to have the SimCity base game. Cities of Tomorrow is only an expansion and on the Origin web-page it is highlighted that you need to own SimCity in order to play the expansion.
https://www.origin.com/irl/en-us/store/simcity/simcity/expansion/simcity-cities-of-tomorrow
For any duplicate purchases, you'll need to contact the retailer.
Hope this helps 