Prevent creating multiple collection items

Really hoping someone can help with this.
So I have 2 collections of Countries and cities. When users sign up, they choose their country and type their city. How do I prevent multiple creation of cities in the same country? Just want to skip creation if city was previously created.
PLEASE HELP.
Thanks@Ben or whoever.

You can use Conditional Actions to only create the record if it does not already exist.

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