Link to the Wikipedia definition and examples Example shown is actually an exception.

Double Check is a very powerful form of a discovered check. Not only do we reveal (discover) an attack on the enemy King from the hidden piece but the piece that moves away also attacks the King. In this case both pieces gain immunity and cannot be captured or blocked since it is impossible to deal with both attacks at the same time. The only defense against a double check is to move the King.

 

In the previous homework:

Instead of recapturing the Bishop on f6 Black plays Nf3++ (double check) and mate. White King has nowhere to go and even though both Queen and Knight are under attack neither can be captured.

In game: Reti,Richard - Tartakower,Saviely [B15] Vienna Vienna, 1910
1.e2-e4 c7-c6 2.d2-d4 d7-d5 3.Nb1–c3 d5xe4 4.Nc3xe4 Ng8-f6 5.Qd1–d3 e7-e5 6.d4xe5 Qd8-a5+ 7.Bc1–d2 Qa5xe5 8.0–0–0 Nf6xe4

Black won an extra Knight. However their King is stuck in the center where it is very unsafe. White sacrifices a whole Queen in order to lure Black King to d8. 9.Qd3-d8+ Ke8xd8 ...and there a double check from Bishop and Rook leads to mate. 10.Bd2-g5+ Kd8-c7 (10... Kd8-e8 11. Rd8# ) 11.Bg5-d8# 1–0