PostgreSQL: How to disable duplicate?

Question

PostgreSQL

How can I prevent a duplicate record if it already exists in the table?

Tried like this:

cur.execute("INSERT INTO likes (postid, like_one, like_two) VALUES (%s, %s, %s) ON CONFLICT (postid)", (postid, 0, 0))

gives an error

Answer 1

There are unique keys, fields and indexes for this.

Answer 2

You can create a unique index for all 3 fields, or an insert trigger.