Blocked attacks cause additional staggering depending how much damage got through the block, so in case of bucklers, you get more stagger if you block without parrying than you would with other shields because their block armor is worse and offset by the parry multiplier. So if you are staggered for any reason you will be subject for double damage, though I'm not sure if in case a blocked attack made you staggered only the leftover damage is doubled or all of it. But any subsequent attacks from other enemies will cause increased damage either way.