diff --git a/src/registry.js b/src/registry.js
index ed28ded..e7c69ea 100644
--- a/src/registry.js
+++ b/src/registry.js
@@ -10,22 +10,30 @@ module.exports = function createRegistry() {
 		_types: new Map(),
 		_traits: new Map(),
 		createType: function (name, schema, options = {}) {
-			let combinedOptions = Object.assign({
-				_registry: this
-			}, options);
+			if (!this._types.has(name)) {
+				let combinedOptions = Object.assign({
+					_registry: this
+				}, options);
 
-			let type = moduleAPI.createType(name, schema, combinedOptions);
-			this._types.set(name, type);
-			return type;
+				let type = moduleAPI.createType(name, schema, combinedOptions);
+				this._types.set(name, type);
+				return type;
+			} else {
+				throw new Error(`A type named ${name} already exists in this registry`);
+			}
 		},
 		createTrait: function (name, schema, options = {}) {
-			let combinedOptions = Object.assign({
-				_registry: this
-			}, options);
+			if (!this._traits.has(name)) {
+				let combinedOptions = Object.assign({
+					_registry: this
+				}, options);
 
-			let trait = moduleAPI.createTrait(name, schema, combinedOptions);
-			this._traits.set(name, trait);
-			return trait;
+				let trait = moduleAPI.createTrait(name, schema, combinedOptions);
+				this._traits.set(name, trait);
+				return trait;
+			} else {
+				throw new Error(`A trait named ${name} already exists in this registry`);
+			}
 		},
 		guardFunction: function (args, returnType, func) {
 			return moduleAPI.guardFunction(args, returnType, func, this);
diff --git a/test/registry.js b/test/registry.js
index e7bd790..d43f461 100644
--- a/test/registry.js
+++ b/test/registry.js
@@ -77,31 +77,31 @@ describe("registry", () => {
 
 		it("should work correctly for traits", () => {
 			let Givable = registry.createTrait("Givable", {
-				from: registry.type("Person"),
-				to: registry.type("Person")
+				from: registry.type("AlsoPerson"),
+				to: registry.type("AlsoPerson")
 			});
 
-			let Person = registry.createType("Person", {
+			let AlsoPerson = registry.createType("AlsoPerson", {
 				name: dm.string(),
 				favouriteGift: registry.trait("Givable").optional()
 			});
 
-			let Gift = registry.createType("Gift", {
+			let AlsoGift = registry.createType("AlsoGift", {
 				description: dm.string()
 			}).implements(Givable, {
 				from: dm.slot(),
 				to: dm.slot()
 			});
 
-			let joe = Person({
+			let joe = AlsoPerson({
 				name: "Joe"
 			});
 
-			let jane = Person({
+			let jane = AlsoPerson({
 				name: "Jane"
 			});
 
-			let flowers = Gift({
+			let flowers = AlsoGift({
 				description: "Flowers",
 				from: jane,
 				to: joe
@@ -119,11 +119,11 @@ describe("registry", () => {
 
 			expect(() => {
 				flowers.to = "not a person";
-			}).to.throw("Expected an instance of Person, got a string instead");
+			}).to.throw("Expected an instance of AlsoPerson, got a string instead");
 
 			expect(() => {
 				flowers.to = flowers;
-			}).to.throw("Expected an instance of Person, got an instance of Gift instead");
+			}).to.throw("Expected an instance of AlsoPerson, got an instance of AlsoGift instead");
 		});
 
 		it("should work correctly for type aliases", () => {
@@ -147,20 +147,20 @@ describe("registry", () => {
 		});
 
 		it("should work correctly for guarded maps and sets", () => {
-			let SomeType = registry.createType("SomeType", {
+			let SomeNewType = registry.createType("SomeNewType", {
 				value: dm.string()
 			});
 
 			let SomeOtherType = registry.createType("SomeOtherType", {
-				things: dm.setOf(registry.type("SomeType")),
-				thingsMap: dm.mapOf(dm.string(), registry.type("SomeType"))
+				things: dm.setOf(registry.type("SomeNewType")),
+				thingsMap: dm.mapOf(dm.string(), registry.type("SomeNewType"))
 			});
 
-			let thingOne = SomeType({
+			let thingOne = SomeNewType({
 				value: "one"
 			});
 
-			let thingTwo = SomeType({
+			let thingTwo = SomeNewType({
 				value: "two"
 			});
 
@@ -186,7 +186,7 @@ describe("registry", () => {
 						["two", thingTwo]
 					])
 				});
-			}).to.throw("Expected an instance of SomeType, got a string instead");
+			}).to.throw("Expected an instance of SomeNewType, got a string instead");
 
 			expect(() => {
 				SomeOtherType({
@@ -199,7 +199,23 @@ describe("registry", () => {
 						["two", "things"]
 					])
 				});
-			}).to.throw("Expected an instance of SomeType, got a string instead");
+			}).to.throw("Expected an instance of SomeNewType, got a string instead");
+		});
+
+		it("should reject duplicate type registrations", () => {
+			registry.createType("DuplicateType", {});
+
+			expect(() => {
+				registry.createType("DuplicateType", {});
+			}).to.throw("A type named DuplicateType already exists in this registry");
+		});
+
+		it("should reject duplicate trait registrations", () => {
+			registry.createTrait("DuplicateTrait", {});
+
+			expect(() => {
+				registry.createTrait("DuplicateTrait", {});
+			}).to.throw("A trait named DuplicateTrait already exists in this registry");
 		});
 	});
 });