Can you have a double flat in diminished triad?

Yes, a double flat in a diminished triad is possible. A diminished triad is composed of three notes, with the interval between the first and second note being a minor third, and the interval between the second and third note also being a minor third. By flattening the second note by two half-steps, or adding a double flat, the diminished triad will have a more dissonant sound.