A Beginners' Guide to Blazor WebAssembly - Preventing Duplicate Records

How to avoid adding duplicate records to the database and display a warning message to the user when an attempt to insert a duplicate is made.
Useful Links:
Video script, including code: blazorcode.uk/countriesandcit...
DB Browser for SQLite: sqlitebrowser.org
Dapper: github.com/DapperLib/Dapper
SQLite Tutorial: www.sqlitetutorial.net
Learn Dapper: www.learndapper.com
Dapper Tutorial - Dynamic Parameters: dapper-tutorial.net/parameter...