duplicate_column column “…” of relation “…” already exists — 42701

Symptoms

An ALTER TABLE … ADD COLUMN (or a CREATE TABLE) tried to add a column whose name already exists on that table. PostgreSQL reports SQLSTATE 42701 (duplicate_column).

• Most common on non-idempotent ADD COLUMN migrations.
• The message names both the column and the relation.
• The table is left unchanged.

What the server log shows

ERROR:  column "status" of relation "orders" already exists

Why PostgreSQL raises this — what the manual says

the ALTER TABLE reference (ADD COLUMN):

“This form adds a new column to the table, using the same syntax as CREATE TABLE.”

Column names are unique within a table. ADD COLUMN checks pg_attribute for an existing live column of the same name and raises 42701 if found, leaving the table untouched.

Common causes

• A migration that already ran being applied again.
• Two migrations adding the same column.
• The column was added manually outside the migration tool.

How to fix it

1. Use ALTER TABLE orders ADD COLUMN IF NOT EXISTS status text;.
2. Check the current columns with \d orders before adding.
3. tuple/" title="Tuple">Record applied migrations so they do not re-run.

Diagnostic query

-- Does the column already exist on the table?
SELECT attname, atttypid::regtype AS type
FROM pg_attribute
WHERE attrelid = 'orders'::regclass AND attname = 'status' AND NOT attisdropped;

Related & next steps

• SQLSTATE 42701 — Duplicate Column (full reference)
• Message: column “…” specified more than once
• Glossary: Column

Reference: PostgreSQL 18 — ALTER TABLE.