PostgreSQL throws the duplicate column error (42701) when you attempt to add a column that already exists or specify the same column name multiple times. Fix it by checking existing columns, using aliases in joins, or leveraging IF NOT EXISTS clauses in migrations.
The "duplicate column" error (error code 42701) occurs when PostgreSQL detects that you are trying to create, add, or reference a column name that already exists in a table. PostgreSQL enforces strict column name uniqueness within each table to maintain unambiguous schema definitions. This error can happen in several scenarios: when using ALTER TABLE ADD COLUMN on a column that already exists, when creating a view with duplicate column names from joined tables, when specifying the same column multiple times in a SELECT statement, or when running migration scripts that attempt to add columns that have already been added.
Before adding a column with ALTER TABLE, query the information_schema to verify it doesn't already exist:
SELECT column_name
FROM information_schema.columns
WHERE table_name = 'your_table' AND column_name = 'your_column';If this returns a row, the column already exists and you should skip the add operation.
When adding a column, use the IF NOT EXISTS clause to make your migration idempotent and prevent errors on re-run:
ALTER TABLE your_table ADD COLUMN IF NOT EXISTS your_column VARCHAR(255);This prevents the duplicate column error if the column was already added in a previous migration run.
When selecting from joined tables that have columns with the same name, use aliases to disambiguate them:
-- Instead of:
SELECT * FROM users u JOIN orders o ON u.id = o.user_id;
-- Use aliases:
SELECT u.id AS user_id, u.name AS user_name,
o.id AS order_id, o.created_at
FROM users u
JOIN orders o ON u.id = o.user_id;This prevents duplicate column names in the result set.
If using CREATE TABLE AS with SELECT from joined tables, explicitly list columns instead of using wildcard selects:
-- Instead of:
CREATE TABLE new_table AS
SELECT t1.*, t2.* FROM table1 t1 JOIN table2 t2 ON t1.id = t2.id;
-- Use explicit column selection:
CREATE TABLE new_table AS
SELECT t1.id AS t1_id, t1.name AS t1_name,
t2.id AS t2_id, t2.description
FROM table1 t1
JOIN table2 t2 ON t1.id = t2.id;If a migration script failed partway through, review it for idempotency issues. Make all schema modifications conditional:
-- Check before dropping
DO $$ BEGIN
IF EXISTS (
SELECT 1 FROM information_schema.columns
WHERE table_name = 'your_table' AND column_name = 'old_column'
) THEN
ALTER TABLE your_table DROP COLUMN old_column;
END IF;
END $$;
-- Add only if not exists
ALTER TABLE your_table ADD COLUMN IF NOT EXISTS new_column INT;Re-run the script after fixing it.
For complex scenarios with multiple duplicate columns, you may need to use PostgreSQL's DO blocks with conditional logic to handle schema changes safely. When working with tools like Flyway, Liquibase, or ORM migrations (Django, Sequelize, Prisma), ensure they are configured to be idempotent so they can be safely re-run. In some cases where a table has been created with duplicates (which normally isn't possible), you may need to recreate the table entirely using CREATE TABLE AS with explicit column selection. PostgreSQL 15+ has better support for IF NOT EXISTS in ALTER TABLE commands, so upgrade if you're on an older version. Always test migrations in a development environment first.
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