After reading this question about the potion that protected the horcrux in the cave I got to wondering. Could multiple people have drank the potion in the cave, thus diluting its effect on any single person, while still permitting the horcrux to be retrieved?
It's probable (nothing explicitly seems to contradict the idea), but likely not the case since it would have been considered unnecessary due to the restrictions on how many wizards could get onto the island via the boat:
"It doesn't look like it was built for two people. Will it hold both of us? Will we be too heavy together?"
Dumbledore chuckled. "Voldemort will not have cared about the weight, but about the amount of magical power that crossed his lake. I rather think an enchantment will have been placed upon this boat so that only one wizard at a time will be able to sail in it."
I suppose so, because the potion was made for drinking, that was the problem. How many most likely didn't matter. Dumbledore just didn't want Harry to drink it just in case it was dangerous. It was more important that Harry lived than him, to Dumbledore. It was the boat that was made to ensure only one wizard, the potion was made to force the "thief/thieves" to drink it.