Double Check

Rg6+

The rook on e6 stands on its own bishop's diagonal. A rook does not attack diagonally, so there is no check yet. It moves to g6 and checks down the g-file, and leaving e6 also opens the bishop's diagonal check. One move gives two checks on two different lines. It is a double check with no other variant.

Re1+

Double Check differs from a discovered check by the number of checking pieces after the move, at least two instead of one. In the counter-example shown here, only the revealed bishop checks, so the position is a discovered check, not a double check.

Without the rook's own check, only the bishop checks. This is a single, plain discovered check and not a double check. The shape is that of a Discovered Check alone, in which the piece leaves the line and the slider behind it alone checks the king. It does not count as a realised discovered attack either. The black king has a free escape on h8 that loses no material and does not walk into mate, so the check gains nothing against best defence. Neither motif is realised here.