How to fix Too many rows in PostgreSQL

BEGIN
  SELECT * INTO STRICT myrec FROM users WHERE email = user_email LIMIT 1;
EXCEPTION
  WHEN TOO_MANY_ROWS THEN
    RAISE EXCEPTION 'Multiple users found with email %', user_email;
END;

-- WRONG: matches multiple users with same email domain
SELECT * INTO STRICT user_rec FROM users WHERE email LIKE '%@example.com';

-- CORRECT: matches exactly one user by primary key
SELECT * INTO STRICT user_rec FROM users WHERE id = user_id;

BEGIN
  -- Without STRICT: assigns first row, ignores others
  SELECT id, name INTO user_id, user_name FROM users WHERE status = 'active';
  
  -- If no rows found, variables are set to NULL
  IF NOT FOUND THEN
    RAISE EXCEPTION 'No active users found';
  END IF;
END;

DECLARE
  myrec RECORD;
BEGIN
  SELECT * INTO STRICT myrec FROM products WHERE sku = product_sku;
EXCEPTION
  WHEN NO_DATA_FOUND THEN
    RAISE EXCEPTION 'Product with SKU % not found', product_sku;
  WHEN TOO_MANY_ROWS THEN
    RAISE EXCEPTION 'Data integrity error: product SKU % is not unique', product_sku;
END;

-- Find duplicate values causing the error
SELECT email, COUNT(*) FROM users GROUP BY email HAVING COUNT(*) > 1;

-- Add unique constraint to prevent future duplicates
ALTER TABLE users ADD CONSTRAINT uq_users_email UNIQUE (email);

-- Or create a unique index if some values can be NULL
CREATE UNIQUE INDEX idx_users_email ON users(email) WHERE email IS NOT NULL;