"Duplicate column name" (ALTER TABLE ADD COLUMN)

You tried to add a column with a name the table already has. Usually a re-run migration — sometimes a sign the column was never actually missing in the first place.

• SQLite: duplicate column name: country
• PostgreSQL: ERROR: column "country" of relation "customers" already exists
• MySQL: Duplicate column name 'country'
• SQL Server: Column names in each table must be unique. Column name 'country' in table 'customers' is specified more than once.

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The cause — almost always a re-run migration

This is the ALTER TABLE sibling of table already exists, and the most common cause is identical: a migration that added this column already ran once, and something re-ran it — a re-deployed migration, a migration tool that lost track of what already applied, or a manual ALTER TABLE run twice by accident.

Check for IF NOT EXISTS support before reaching for it

Unlike CREATE TABLE IF NOT EXISTS, support for the equivalent on ALTER TABLE ADD COLUMN isn't universal — check before relying on it:

• PostgreSQL (9.6+): supports it directly — ALTER TABLE customers ADD COLUMN IF NOT EXISTS country TEXT;
• MySQL (8.0.29+): supports it directly, same syntax.
• SQL Server: no IF NOT EXISTS on ADD COLUMN — guard it explicitly instead:

  IF NOT EXISTS (
    SELECT 1 FROM sys.columns
    WHERE object_id = OBJECT_ID('customers') AND name = 'country'
  )
  ALTER TABLE customers ADD country VARCHAR(100);

• SQLite: no IF NOT EXISTS on ADD COLUMN either — same pattern, check PRAGMA table_info first:

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Since that returns 1, the real migration tooling for this table should skip re-adding country — which is exactly what a proper migration framework's own tracking table is for; this pragma check is what to reach for only when writing raw, ad-hoc DDL without one.

The other case: the column was never actually missing

If this error shows up unexpectedly — not from a known re-run, but from a migration you believed was adding a genuinely new column — stop and check whether an earlier migration (or a manual change) already added it under the same name, possibly with a different type or constraint than the one you're about to add. Blindly working around the error here can leave you with the wrong column definition and no error to tell you.