In Postgresql, force unique on combination of two columns

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I would like to set up a table in PostgreSQL such that two columns together must be unique. There can be multiple values of either value, so long as there are not two that share both.

For instance:

CREATE TABLE someTable (
    id int PRIMARY KEY AUTOINCREMENT,
    col1 int NOT NULL,
    col2 int NOT NULL
)

So, col1 and col2 can repeat, but not at the same time. So, this would be allowed (Not including the id)

1 1
1 2
2 1
2 2

but not this:

1 1
1 2
1 1 -- would reject this insert for violating constraints

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