tournament rule question

[Rocco_835]

This situation happened last night at a tournament I was running and I honestly didn't know the correct rule...but here goes.

Blinds at 1000/2000, the player under the gun (call him player A) raised to 5000 total. The player behind him (player B) goes all-in for 7500 total. The button (player C) calls the 7500, the blinds fold and the action goes back to the original raiser, player A, who made it 5000 originally. He reraises himself all-in, which was about 8500 total.

Some of the people at the tournament were telling me that the original raiser (player A) can't re-raise because when player B put himself all-in for only 7500 total...it didn't qualify as an actual raise because he couldn't double the previous raise. So they were trying to tell player A that his only option was to flat call the extra 2500 preflop.

Personally, I think this sounds crazy. I've never heard of that rule and as far as I understand it...when you play No-Limit poker, there is no point in which you can tell someone that they aren't allowed to raise.

Has anyone encountered this before or does anyone know of this rule or where to find it?

Thanks

-Rocco