TLDR

Ionic spark doesn't stack. If you have two ionic sparks on your team, enemy champions that cast spells will still do 200 true damage every time.

Full explanation

Ionic spark is an item that causes enemy champions to instantly take 200 true damage every time they cast a spell. Some items in TFT stack (i.e. putting multiple items on a champion causes the effects of the item passives to be multiplied). Ionic spark doesn't stack per champion AND per team. That means that you should always build at most one ionic spark for your team.