[Update: From Software and Sony have released patch 1.02 for Bloodborne which puts the kibosh on this exploit.]

Original story:

Bloodborne is a pretty hard game, which is why players should try to use anything and everything in order to gain an advantage over the enemies you’ll face. Sure, you could follow our Boss or co-op guides, but a newly-discovered exploit can give Hunters the unfair advantage they so desperately need.

The exploit allows you to create a duplicate of any item in a pretty simple way. Before we get into it, we should note that performing this exploit will go against the real Bloodborne experience you’ve paid for, so only consider this exploit if you really need the help.

The first thing you’ll need to do is create an alternate character and progress them through the game far enough so they gain access to the first lantern in Central Yharnam. Once you do, transport yourself to the Hunter’s Dream and then log back into your main character. Once there, you’ll want to empty out your storage chest.

With your storage chest emptied with your main character, go to your alternate character and buy a pebble. Store the pebble you just purchased into your alternate’s chest, making sure that’s the only thing in there. Now go back to your main character and decide what it is you want to duplicate, noting that only items you can have multiples of are the only ones that can be duped. That means no duping of weapons.

Once you have in mind what you want to duplicate, put one of that item in your storage chest as your main character. Now go and purchase a lot of pebbles, making sure to reach the limit you can carry, which is 20. Once you reach the limit of pebbles you can carry, the extra pebbles will show up in your chest as duplicates of the item you currently have in there. For example: if you purchase 5 extra pebbles that you can’t carry, those pebbles will be transferred to your chest and will show up as 5 of whatever item is in there.

As with all exploits, we're sure this will be addressed in a future update to Bloodborne, so don't ponder whether or not you should do it for too long. You'll probably miss the boat on this one pretty soon.