I’m not aware of it being possible to create two separate CityStrides accounts for a single Garmin account (or any tracking service), so it sounds like you created duplicate Garmin accounts as well. In that case - How do I fix accidentally creating two accounts at the same tracking service? - should help you out.
Just a quick note that could be helpful - the sign in / sign up process is exactly the same every time regardless of whether or not you already have a CityStrides account.
If you’re not logged into CityStrides, there’s no difference between sign up/in until the very last step (where you’re returning to CityStrides after allowing access in the tracking service) where my code sees that this account already exists & logs you in.