It's a double-sz. So it's pronounced as a long sz.
If you double digraph (and trigraph) letters, you double only the first character: gy - ggy, dz - ddz, zs - zzs, and so on.

Though watch out if it's a compound word like karosszék (from karos + szék). You would pronounce that like s + sz, but since the sibilants are tending to blend together, you end up with a long sz instead.

"hibás" singular, "hibásak" plural