After `insert on conflict replace` record is either missing from the table or gets duplicated

Found in test_insert_on_conflict.

Example with empty result set:

        dml = i1.sql(
            """
            insert into "t" values (1, 2) on conflict do replace
        """
        )
        assert dml["row_count"] == 1
    
        data = i1.sql(
            """select * from "t"
        """
        )
>       assert data["rows"] == [[1, 2]]
E       assert [] == [[1, 2]]
E         Right contains one more item: [1, 2]
E         Full diff:
E         - [[1, 2]]
E         + []

Full test log: fail1.log

Example with new value being duplicated:

        dml = i1.sql(
            """
            insert into "t" values (1, 2) on conflict do replace
        """
        )
        assert dml["row_count"] == 1
    
        data = i1.sql(
            """select * from "t"
        """
        )
>       assert data["rows"] == [[1, 2]]
E       assert [[1, 2], [1, 2]] == [[1, 2]]
E         Left contains one more item: [1, 2]
E         Full diff:
E         - [[1, 2]]
E         + [[1, 2], [1, 2]]

Full test log: fail2.log

Unfortunately it doesnt reproduce every time. In total I've got 3 failures, the first one I've got with pytest -n20. Apparently the chance of reproduction is higher when system is under load. Best recipe I've got is to run taskset -c 0 bash -c "while pytest -k insert_on_conflict; do true; done" in one terminal and taskset -c 0 cargo build --release on random project in another. Using something like stress utility should work too but I had no luck with it.