You can’t do this in exactly the way you’re describing (validating input to a new row before accepting the input).
But you could do something with an intermediate table. How it would work is you’d have another table, maybe called Add Keyword.
It would have 1 row.
Its columns would be keyword (text), a submit button, and a message column.
Submit button flow:
- If keyword already exists in the main table, set message column to “this keyword already exists”
- Otherwise:
- Add the keyword row to the main table
- Set message column to “Keyword added”
- Delete the keyword from the add keyword table
For bonus points
- Clear the message field after a period of time (you could explore automations, or some hidden columns with raw message, message timestamp, and timer)
- Have separate version of this submit form per user of the doc