Slow duplicate deletes

I have a large table (20M records) but mostly short text fields. There are
duplicates that I am trying to remove.  I have a bigseriel index that I
unique, but duplicates in another field.

I have an 8 core, 12GB memory computer with RAID disks.

This request has been running for 70 hours (is it safe to kill it?).

How can I make this run faster? This is a one time processing task, but it
is taking a long time.

DELETE FROM portal.metadata
WHERE idx NOT IN
(
    SELECT MIN(idx)
    FROM portal.metadata
    GROUP BY "data_object.unique_id"
);

CREATE TABLE
    metadata
    (
        data_object.unique_id CHARACTER(64) NOT NULL,
        size_bytes BIGINT,
        object_date TIMESTAMP(6) WITHOUT TIME ZONE,
        object_type CHARACTER VARYING(25),
        classification CHARACTER VARYING(7),
        object_source CHARACTER VARYING(50),
        object_managed_date TIMESTAMP(6) WITHOUT TIME ZONE,
        clevel INTEGER,
        fsctlh CHARACTER VARYING(50),
        oname CHARACTER VARYING(40),
        description CHARACTER VARYING(80),
        horizontal_datum CHARACTER VARYING(20),
        do_location CHARACTER VARYING(200),
        elevation_ft INTEGER,
        location USER-DEFINED,
        idx BIGINT DEFAULT nextval('portal.metadata_idx_seq'::regclass) NOT
NULL,
        bbox CHARACTER VARYING(160),
        CONSTRAINT MetaDataKey PRIMARY KEY (idx)
    )


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