Mutual Follows Query

I have a table with follower and followee (where the follower follows the followee). How do you return a table of mutual follows?

Return each mutual relationship once, even though the source contains two directional rows. Ignore incomplete rows where either user ID is NULL.

Output

1. One row per mutual pair, with columns follower_id and followee_id.
2. The lower ID must appear in follower_id, and the higher ID in followee_id.
3. Order by follower_id, then followee_id ascending.