I've run into bob's head 3 times in a single run. I picked it up every time and even used it in an attempt to remove it from the pool. Why does it keep popping up even though I've picked it up? These 3 occurrences happened all on different levels.
Bob’s head transcends item pools
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Related Solutions
The wiki is partially correct, but not entirely. From my own personal exploration, I can confirm that an angel/devil room WILL spawn after EVERY boss fight where one can, however it does NOT block damage from above.
It turns enemies on screen into poop. Does not work on bosses.
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Best Answer
Bob's Rotten Head appears in two item pools- the Item Room pool and the Gold Chest Pool. Removing it from one pool doesn't remove it from the other so it is possible, if you're unlucky, to get it twice in a run- once from each.
As well as this, Bob's Rotten Head can drop from Sloth/Mega Sloth. This is independent from item pools, so you can get it a third time this way.