It doesn't seem intended, but hopefully GGG won't fix this. It's possible that this glitch may be deleted though, along with the items you duplicated, so I suggest using the poe items/currency as soon as you get them. There's a chance you may be banned for this, and if that happens, I apologise. You don't need a path of exile currency tab for this, though it will be very useful to have one.
How to do it?
It's easily done in 2 steps. The second one is unreliable though, it's mostly RNG.
Step 1: You need to get to Act 5 and defeat Innocence.
If you've already passed Act 5, it's fine, you can still warp back and complete this. Once you beat Innocence, Sin will appear and a cutscene will play. You need to make sure you watch the entire cutscene. If you already beat Innocence, you can still talk to Sin and it will play the cutscene.
Immediately after playing this cutscene, you need to take a portal out of the area, and take a way point to the Act 1 area: The Coast.
Step 2: Walk across The Coast until you encounter a beast.
The beast's name is all glitched, so you will easily recognise it when you see it. You have two options when the beast spawns, you can kill it, or you can capture it, either option will still duplicate your items.
I'd not advise this if you were a racer, or plan to get on the leaderboards for this league, as it will be obvious that you used this glitch, since using this will also cause your name to be be glitched out on the leaderboards.
Only use this if you're too lazy to grind Div Cards/Currency but you still want to have fun. Or to get 124 Leveling Books on your character.