lueddemanniana is known to have a "horned" or "winged" column, two points on the sides, whereas other species usually have a rounder column.
A double sheath is a sheath inside of a sheath - like an extra fold. You can shine a light from the back of the sheath to see if it's just the one, or if there's a second.
Sheaths are like thin, small, modified leaves at the center of a cattleya leaf. The protect the buds as they start coming out. Some cattleyas do not have them, most have one and a few species have one inside the other.
On this photo the dry part that the flower stalk is comong out of ia the sheath ( this is a single sheath)