Teamfight Tactics is a completely different kind of game than League of Legends. It may feature the same champions and many of the same items, but when it comes to the ins and outs, it’s another story.
TFT is in its beta phase and its UI still leaves much to be desired. It’s difficult to figure out each item, their stats, and how they synergize with each other. At the moment it’s a guessing game. You’ll have to do some experimenting to figure out what’s what, but know this: Some items stack in TFT, and some don’t.
It’s much easier to list the items that don’t stack, than the items that do stack. Most of the items in the game stack in one form or another, but there are a few exceptions. The Spatula-based items, including Youmuu’s Ghostblade, Knight’s Vow, Frozen Mallet, Yuumi, and Darkin don’t stack. You simply can’t allocate two of the same Class or Origin to the same unit. And even if you could, it would be a waste of items.
One of the strongest defensive items in the game that doesn’t stack, is Phantom Dancer. Adding another PD changes nothing. The same goes for Redemption. On death, it heals 1000 damage to all nearby allies. This cannot be duplicated on the same target. There’s absolutely nothing wrong with allocating Redemption onto another unit, however.
Almost every other item in the game stacks. It doesn’t necessarily mean they’re worth stacking, though. In some cases, item stacking is a powerful tactic, helping bolster up a unit to carry your team. But in others, it’s a waste of resources. In the case of Spear of Shojin, the strongest offensive item in the game, stacking is often a good strategy. For champions like Pyke, Aurelion So, Lulu or even Ashel, it’s ideal. The more ultimates you can dish out, the better.
Overall, item stacking is important to TFT. You shouldn’t be looking to stack each and every item, but in the cases of the powerful few, it’s an intelligent option.