Prevent attribute items from being embedded if there is already an attribute with the same key

module/data/Actor/player.mjs

@@ -59,9 +59,9 @@ export class PlayerData extends foundry.abstract.TypeDataModel {
 	};
 	// #endregion Lifecycle
 
-	// #region Getters
+	// #region Methods
 	get hasAttributes() {
 		return Object.keys(this.attr).length > 0;
 	};
-	// #endregion Getters
+	// #endregion Methods
 };

module/data/Item/attribute.mjs

@@ -1,4 +1,5 @@
 export class AttributeItemData extends foundry.abstract.TypeDataModel {
+	// #region Schema
 	static defineSchema() {
 		const fields = foundry.data.fields;
 		return {
@@ -32,8 +33,33 @@ export class AttributeItemData extends foundry.abstract.TypeDataModel {
 			}),
 		};
 	};
+	// #endregion Schema
 
+	// #region Lifecycle
+	async _preCreate(data, options, user) {
+
+		// Prevent duplicate Attribute keys from existing on a single Actor
+		if (this.parent.isEmbedded) {
+			const attr = this.parent.parent?.getAttribute(this.key);
+			if (attr) {
+				ui.notifications.error(
+					`taf.notifs.error.duplicate-attribute-key`,
+					{
+						localize: true,
+						format: { key: this.key },
+					},
+				);
+				return false;
+			};
+		};
+
+		return super._preCreate(data, options, user);
+	};
+	// #endregion Lifecycle
+
+	// #region Methods
 	get isRange() {
 		return this.max !== null;
 	};
+	// #endregion Methods
 };