Why does Add Unique add a struct that's apparently already there?

BP struct equality compares all members. Two structs differ in any member — even an unused field with a slightly different float — are unequal. De-dup by key with Find by Predicate first, then Add.

Can I override struct equality?

In C++, define operator== and a Has Native Compare hint. From BP-only code, write a Function that takes the array + candidate and de-dupes by your own equality.

Use Set instead of Array?

For primitive types and unique objects, Set deduplicates by GetTypeHash. Structs need GetTypeHash overload to be Set-storable; otherwise stick with Array + custom de-dup.

Fix: Unreal BP Add Unique Adds Duplicate Struct

Quick answer: Use a key field. Find by predicate matching the key. Add only when not found.

Inventory uses an FInventoryItem struct with ItemID + Quantity. Add Unique with same ID but different Quantity adds again because Quantity differs.

The Fix

// BP function: AddOrUpdateInventory
Inputs: Items (Array of FInventoryItem ref), New (FInventoryItem)

For each Item in Items:
  Branch: Item.ItemID == New.ItemID
    True → Set Item.Quantity = Item.Quantity + New.Quantity; return
Add(Items, New)   // truly new ID

De-dup logic lives in your function. Add Unique BP node only handles primitives; structs need explicit key handling.

C++ Native Compare

USTRUCT(BlueprintType)
struct FInventoryItem {
    GENERATED_BODY()
    UPROPERTY(EditAnywhere) FName ItemID;
    UPROPERTY(EditAnywhere) int32 Quantity;

    bool operator==(const FInventoryItem& o) const {
        return ItemID == o.ItemID;   // key only
    }
};

Verifying

Add same ItemID twice with different quantities: list keeps one entry, quantity sums. Different ItemID: separate entry.

“Key on identity. De-dup explicitly. Inventory clean.”

Related Issues

For BP Interface defaults, see BP Interface. For replication scope, see replication.

Identity key. Find then add.