Let's say duplicate key value violates unique constraint is being returned when attempting to insert a record into a Postgres table.

psql -U postgres_user -c "INSERT INTO "my_table" ("id", "name") VALUES ('12345', 'john.doe')"
ERROR:  duplicate key value violates unique constraint "unique_name"
DETAIL:  Key (name)=(john.doe) already exists.

Notice in this example that the "name" column has UNIQUE CONSTRAINT meaning each record in my_table must have a unique name.

~]$ psql --username postgres_user --dbname my_database
my_database=# \d my_table
                              Table "public.my_table"
        Column         |           Type           | Collation | Nullable | Default 
-----------------------+--------------------------+-----------+----------+---------
 id                    | uuid                     |           | not null | 
 name                  | text                     |           |          | 
Indexes:
    "primary_key" PRIMARY KEY, btree (id)
    "unique_name" UNIQUE CONSTRAINT, btree (name)

The most likely issue here is that there is already a record in my_table with name john.doe.

~]$ psql --username postgres_user --dbname my_database --command "select * from my_table where name = 'john.doe'" -x
-[ RECORD 1 ]---------+-------------------------------------
id                    | 12345
name                  | john.doe