Why not create a Country entity in your domain model and have one record for each country? Then you can create an association between your entity and the Country they are located in. The drop-down based on the association will then only show your distinct country records.
Edit: The first thing you need to do is a Retrieve action from the Database and get the First record only where the Country name = your value, like:
Then use the record retrived in a Change ation to set the value of the association, like:
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