An item from heroic Throne of Thunder has another appearance and is not unique - however it does not clear when lookalike item is obtained. I have the completionist mode off and only look to gain one item to unlock all apperances.

Item: Hypersensitive Sollerets (Heroic-Thunderforged) - Ra-den
Item in-game number: 95010

Shared appearance with:

(H) Ice-Scored Treads - item in-game number: 96460

they're not actually shared appearances; they're visually quite different. just the color scheme is shared. there are a handful of tier/lookalike sets that have like one piece that is shaped differently or has a slightly different visual effect that causes them to not be shared appearances. if you inspect the items in-game you can see they are not the same: