Patton Oswalt will return to Agents of S.H.I.E.L.D. in the episode "Hot Potato Soup," airing January 31, according to Entertainment Weekly. In the episode, Oswalt will reprise his dual role as brothers Sam and Billy Koenig - and they'll reveal their family secret.
As for what the secret is, given that viewers have already met three identical Koenig brothers - the aforementioned Sam and Billy, as well as Eric - and that there may be as many as 13 Koenig brothers spread throughout S.H.I.E.L.D., it seems a safe bet that their "family secret" is that they're all LMD's, which have become a major plot point in the current season.
Agents of S.H.I.E.L.D. airs Tuesday nights on ABC. "Hot Potato Soup" will air on January 31.