@techddx You should create an array in the script editor, as the user is entering the value in the table field you can add that value to the array, and before inserting you can check if the value exists in the array using 'includes'.
const greetings = ['hi','hello'];
greetings.includes('hi'); // => true so you can prompt error message
greetings.includes('hey'); // => false, add this to greetings => ['hi','hey']