Riddle #2

Seeing double

While pacing up and down the carriage pondering on the strange announcement, you look outside and see T3 approaching.

Your attention shifts to a poster on the wall next to the window. The poster has at least 40 different butterflies on it. You quickly figure out that they must be the different species found in the Butterfly Garden. On closer inspection, you observe that each butterfly is assigned a letter or number.

On the poster you notice the faintest handwriting, as if someone has scribbled over it with pencil:

When you see double,
you’re out of trouble.