Incident brief
Column specified more than once
A CREATE TABLE listed the same column name twice. PostgreSQL refuses to build a table that has two columns with the same name.
In 10 seconds
- What
- Column specified more than once
- What triggers it
- Write a CREATE TABLE that lists the same column name (here id) two times.
- The fix
- Give every column a distinct name (here, rename the second id to user_id).
- Proof
- Reproduced on PostgreSQL 16.14 → A CREATE TABLE that listed id twice was rejected with SQLSTATE 42701 (duplicate_column). No table was created and the session stayed usable.
The fix
What to do right now
The immediate, application-level response to this error.
- Give every column a distinct name (here, rename the second id to user_id).
- When generating DDL programmatically, de-duplicate the column list before emitting CREATE TABLE.
- Watch for a column that collides with one pulled in by LIKE or INHERITS — the merge rules can surface the same name.
-- every column has a distinct name
CREATE TABLE reg_users (user_id int, email text);Diagnose
See it live on the server
Run these against the affected instance to confirm the diagnosis before you act.
Standard triage — not specific to this error
These are canonical PostgreSQL system-catalog queries, shown as SQL to run. No sample output is attached because this is general triage, not a captured lab transcript.This SQLSTATE does not have an error-specific live snapshot yet. These are the canonical system-catalog queries you run against the affected server to see the problem in real time — standard triage, not a reproduced transcript.
What is running right now
Active backends, how long each has been running, and what it is waiting on.
SELECT pid,
state,
wait_event_type,
wait_event,
now() - query_start AS running_for,
left(query, 80) AS query
FROM pg_stat_activity
WHERE state <> 'idle'
AND pid <> pg_backend_pid()
ORDER BY running_for DESC NULLS LAST;Who is blocking whom
Turn raw blocking PIDs into the actual queries on both sides of the wait.
SELECT blocked.pid AS blocked_pid,
blocked.query AS blocked_query,
blocking.pid AS blocking_pid,
blocking.query AS blocking_query
FROM pg_stat_activity AS blocked
JOIN LATERAL unnest(pg_blocking_pids(blocked.pid)) AS b(pid) ON true
JOIN pg_stat_activity AS blocking ON blocking.pid = b.pid
WHERE cardinality(pg_blocking_pids(blocked.pid)) > 0;Locks that are still waiting
Every lock a backend has requested but not yet been granted.
SELECT l.pid,
l.locktype,
l.mode,
l.granted,
COALESCE(c.relname, l.transactionid::text) AS object
FROM pg_locks l
LEFT JOIN pg_class c ON c.oid = l.relation
WHERE NOT l.granted
ORDER BY l.pid;Why it happens
What PostgreSQL is telling you
The mechanism behind the error, grounded in the official manual — not paraphrased.
PostgreSQL 16 Documentation — Appendix A. PostgreSQL Error Codes
All messages emitted by the PostgreSQL server are assigned five-character error codes that follow the SQL standard's conventions for “SQLSTATE” codes.Read the full section on postgresql.org →
The CREATE TABLE with a repeated column name
The column list names id twice. PostgreSQL cannot create a relation with two identically named columns, so it raises SQLSTATE 42701 and creates nothing.The session is unaffected by the parse error
Because the DDL failed at parse time outside a transaction block, the connection is untouched and the next statement runs normally.Reproduce & verify
A real, single-session PostgreSQL reproduction
A literal transcript of SQL run against a live PostgreSQL instance in an isolated lab — the commands below are exactly what was executed.
- 1Write a CREATE TABLE that lists the same column name (here id) two times.
- 2PostgreSQL rejects the DDL with SQLSTATE 42701 before the table is created.
- 3No table is created; the session is otherwise unaffected.
One client: a single CREATE TABLE statement whose column list repeats the name id.
-- A duplicate column name is a parse error, so no tables need to exist first.
SELECT 'no prior schema needed' AS setup_note;CREATE TABLE reg_users (id int, email text, id int);SELECT 'ok' AS session_after_error;What PostgreSQL actually returned
setup_note
------------------------
no prior schema needed
(1 row)ERROR: column "id" specified more than once session_after_error
---------------------
ok
(1 row)The corrected DDL — with a distinct second column name — was executed and produced a working table and row, confirming the only defect was the repeated name.
Rename the duplicate column to user_id, create the table, and insert a row.
Without this
Above: two columns named id raise SQLSTATE 42701.
With this, tested
Below: distinct column names create the table and accept a row.
What Pro unlocks here
- The exact prevention SQL — copy-paste ready
- Raw psql output captured from the Docker lab
- A senior-DBA action list to take it further
- Live monitoring queries to catch it in production
- The deeper audit: fix-that-fails counterexample, GUC before/after, server-log evidence
Related & next steps
Follow the thread
Everything this error touches — jump straight to the sibling error, term, runbook, or parameter.
Verification
- Last verified
- 2026-07-19 (isolated lab, PostgreSQL 16.14)
- Reviewed by
- Verified against PostgreSQL 16.14 in an isolated lab environment
- Audit status
- reviewed