It seems the need for Airtable to have a system to prompt the user if a duplicate entry has been submitted would be a main requirement for users, particularly those using forms. For example, a user registration form, ensuring only a unique email had been used. I would imagine it would either be based on the primary column or even better, the option to tell Airtable which columns should not allow duplicate entries.
At the moment I have to pay miniextensions over $400 a year just to have this function - a great product, but something I feel Airtable should include as standard. After-all, why should I have to use such plugins to achieve what I would deem basic requirements of a database type system like Airtable? 🙂