"duplicate key value violates unique constraint" (Postgres error 23505)

What it means

You tried to add a row with a value that must be unique, like an id, email or username, and a row with that value already exists. The error message names the constraint, which tells you which column it is.

Why AI tools cause it

• The insert runs twice: a double click, or a React effect firing twice in development.
• A "create profile" step on every login, not just the first, so the second login fails.
• A database trigger already created the row (common with Supabase profile tables), then your code tries to insert it again.
• Ids typed in by hand in seed data, so the automatic counter later hands out an id that's already taken.

The fix

1. Read the constraint name in the error, e.g. profiles_pkey or users_email_key, to see which column clashed.
2. If the row might already exist, use an upsert instead of an insert: .upsert(row, { onConflict: 'id' }) in Supabase.
3. Disable submit buttons while saving so one click means one insert.
4. If you have a trigger that creates profiles, remove the duplicate insert from your app code.

Paste this into your AI

My insert fails with 'duplicate key value violates unique constraint "<constraint name>"'. Find where this row is created, check whether a database trigger or a second code path also creates it, and change the code to an upsert or a check-then-insert so it can safely run more than once. Also stop double submits on the form.

Still broken?

If it's the primary key on a table you seeded by hand, the id counter is behind. Ask your AI for the setval SQL to move the sequence past your highest id.