How to avoid "ON CONFLICT DO UPDATE command cannot affect row a second time" error in WITH statement

I have two tables: _{demo at db<>fiddle}

1. Table keywords has two columns id and v (which hold the keyword's value)

   create table keywords(
     id int generated always as identity primary key
    ,v text unique);
   

2. Table main has, among others, two columns that are each a foreign key into the keywords table's id, named key1_id and key2_id

   create table main(
     id int generated always as identity primary key
    ,key1_id int references keywords(id)
    ,key2_id int references keywords(id));
   

Now, I want to insert pairs of keywords (key1 and key2 as $1 and $2) into the main table, like this:

WITH key1 AS (INSERT INTO keywords (v) 
              VALUES ($1) 
              ON CONFLICT (v) DO UPDATE
              SET v = EXCLUDED.v
              RETURNING id),
     key2 AS (INSERT INTO keywords (v) 
              VALUES ($2) 
              ON CONFLICT (v) DO UPDATE
              SET v = EXCLUDED.v
              RETURNING id)
INSERT INTO main (key1_id, key2_id)
SELECT key1.id, key2.id
FROM key1, key2
RETURNING id

Basically, the two keys often have recurring values, so I use the keywords table to keep a unique set of them (mostly for storage optimization, as I have millions of rows with them).

But if $1 and $2 have identical values, I get this error, whereas there's no issue if they're different:

pg_query_params(): Query failed: ERROR: ON CONFLICT DO UPDATE command cannot affect row a second time
HINT: Ensure that no rows proposed for insertion within the same command have duplicate constrained values.

The goal is that both key1_id and key2_id point to the correct row in keywords based on the values passed as $1 and $2, even if they're both the same.

How do I modify the SQL statement (ideally, it should remain a single one) so that I can insert these keys without getting the error?

I am using Postgresql 16.9.

You can do exactly what the hint suggests:

Ensure that no rows proposed for insertion within the same command have duplicate constrained values.

Reduced to a single insert, with a union making sure duplicate insert never happens and coalesce() just repeating key1.id in that case:
_{demo at db<>fiddle}

WITH keys AS (INSERT INTO keywords (v) 
              SELECT $1
              UNION--deduplicates
              SELECT $2
              ON CONFLICT (v) DO UPDATE
              SET v = EXCLUDED.v
              RETURNING id)
INSERT INTO main ( key1_id, key2_id )
SELECT min(id), max(id)
FROM keys
RETURNING id;