CREATE TABLE forms (
    id serial PRIMARY KEY,
    account INT NOT NULL,
    author text,
    type INT NOT NULL
);
INSERT INTO  forms(account, author, type) VALUES 
(1, 'Иванов', 11),
(2, 'Сидоров', 11),
(3, 'Сергеев', 12)

(1, 'Иванов', 11),
(1, 'Сидоров', 11)

(1, 'Иванов', 12),
(1, 'Сидоров', 12)

CREATE UNIQUE INDEX unique_account_type_for_type_11 ON forms (account, type) WHERE type = 11;