PostgreSQL "duplicate key violation" with SEQUENCE

[Issue resolved. See Answer below.]

I have just encountered a series of “duplicate key value violates unique constraint” errors with a system that has been working well for months. And I cannot determine why they occurred.

Here is the error:

org.springframework.dao.DuplicateKeyException: PreparedStatementCallback;
SQL [
INSERT INTO transaction_item 
  (transaction_group_id, transaction_type, start_time, end_time) VALUES 
  (?, ?::transaction_type_enum, ?, ?)
];
ERROR: duplicate key value violates unique constraint "transaction_item_pkey"
  Detail: Key (transaction_id)=(67109) already exists.;

Here is the definition of the relevant SEQUENCE and TABLE:

CREATE SEQUENCE transaction_id_seq AS bigint;

CREATE TABLE transaction_item (
  transaction_id bigint PRIMARY KEY DEFAULT NEXTVAL('transaction_id_seq'),
  transaction_group_id bigint NOT NULL,
  transaction_type transaction_type_enum NOT NULL,
  start_time timestamp NOT NULL,
  end_time timestamp NOT NULL
);

And here is the only SQL statement used for inserting to that table:

INSERT INTO transaction_item
  (transaction_group_id, transaction_type, start_time, end_time) VALUES 
  (:transaction_group_id, :transaction_type::transaction_type_enum, :start_time, :end_time)

As you can see, I’m not explicitly trying to set the value of transaction_id. I've defined a default value for the column definition, and using that to fetch a value formthe SEQUENCE.

I have been under the impression that the above approach is safe, even for use in high-concurrency situation. A SEQUENCE should never return the same value twice, right?

I’d really appreciate some help to understand why this has occurred, and how to fix it. Thank you!