I imagine any number higher than 1 may have double characters. I realize the test is intended to showcase a theoretical party of good composition, but at the same time, it must reasonably accommodate multiples.
If a party of all rogues chews through the playtest twice as fast as a normal party, there might be issues. If throwing an extra wizard into a normal party causes an appropriately adjusted adventure to crumble into ruin, there might be an issue.
So, ideally you'll double up, and playtesting with 6, 2 fighters, 1 wizard, 2 clerics and a rogue will be just as good as a fighter, a rogue, a wizard, a cleric and some 5th class.