You can create/modify index using IGNORE_DUP_KEY.
DROP INDEX ix_test_column ON test; -- if exists
CREATE UNIQUE INDEX ix_test_column
ON test(unique_column)
WITH (IGNORE_DUP_KEY = ON);
INSERT INTO test (unique_column, other_column)
VALUES
('one', 'mary'),
('two', 'john'),
('one', 'jack');