diff --git a/src/vs/workbench/api/common/configurationExtensionPoint.ts b/src/vs/workbench/api/common/configurationExtensionPoint.ts
index 75476f2f82d..b3d1b380775 100644
--- a/src/vs/workbench/api/common/configurationExtensionPoint.ts
+++ b/src/vs/workbench/api/common/configurationExtensionPoint.ts
@@ -159,6 +159,8 @@ configurationExtPoint.setHandler((extensions, { added, removed }) => {
 		configurationRegistry.deregisterConfigurations(removedConfigurations);
 	}
 
+	const seenProperties = new Set<string>();
+
 	function handleConfiguration(node: IConfigurationNode, extension: IExtensionPointUser<any>): IConfigurationNode[] {
 		const configurations: IConfigurationNode[] = [];
 		let configuration = objects.deepClone(node);
@@ -176,6 +178,60 @@ configurationExtPoint.setHandler((extensions, { added, removed }) => {
 		return configurations;
 	}
 
+	function validateProperties(configuration: IConfigurationNode, extension: IExtensionPointUser<any>): void {
+		let properties = configuration.properties;
+		if (properties) {
+			if (typeof properties !== 'object') {
+				extension.collector.error(nls.localize('invalid.properties', "'configuration.properties' must be an object"));
+				configuration.properties = {};
+			}
+			for (let key in properties) {
+				const message = validateProperty(key);
+				if (message) {
+					delete properties[key];
+					extension.collector.warn(message);
+					continue;
+				}
+				if (seenProperties.has(key)) {
+					delete properties[key];
+					extension.collector.warn(nls.localize('config.property.duplicate', "Cannot register '{0}'. This property is already registered.", key));
+					continue;
+				}
+				const propertyConfiguration = properties[key];
+				if (!isObject(propertyConfiguration)) {
+					delete properties[key];
+					extension.collector.error(nls.localize('invalid.property', "configuration.properties property '{0}' must be an object", key));
+					continue;
+				}
+				seenProperties.add(key);
+				if (propertyConfiguration.scope) {
+					if (propertyConfiguration.scope.toString() === 'application') {
+						propertyConfiguration.scope = ConfigurationScope.APPLICATION;
+					} else if (propertyConfiguration.scope.toString() === 'machine') {
+						propertyConfiguration.scope = ConfigurationScope.MACHINE;
+					} else if (propertyConfiguration.scope.toString() === 'resource') {
+						propertyConfiguration.scope = ConfigurationScope.RESOURCE;
+					} else if (propertyConfiguration.scope.toString() === 'machine-overridable') {
+						propertyConfiguration.scope = ConfigurationScope.MACHINE_OVERRIDABLE;
+					} else if (propertyConfiguration.scope.toString() === 'language-overridable') {
+						propertyConfiguration.scope = ConfigurationScope.LANGUAGE_OVERRIDABLE;
+					} else {
+						propertyConfiguration.scope = ConfigurationScope.WINDOW;
+					}
+				} else {
+					propertyConfiguration.scope = ConfigurationScope.WINDOW;
+				}
+			}
+		}
+		let subNodes = configuration.allOf;
+		if (subNodes) {
+			extension.collector.error(nls.localize('invalid.allOf', "'configuration.allOf' is deprecated and should no longer be used. Instead, pass multiple configuration sections as an array to the 'configuration' contribution point."));
+			for (let node of subNodes) {
+				validateProperties(node, extension);
+			}
+		}
+	}
+
 	if (added.length) {
 		const addedConfigurations: IConfigurationNode[] = [];
 		for (let extension of added) {
@@ -196,54 +252,6 @@ configurationExtPoint.setHandler((extensions, { added, removed }) => {
 });
 // END VSCode extension point `configuration`
 
-function validateProperties(configuration: IConfigurationNode, extension: IExtensionPointUser<any>): void {
-	let properties = configuration.properties;
-	if (properties) {
-		if (typeof properties !== 'object') {
-			extension.collector.error(nls.localize('invalid.properties', "'configuration.properties' must be an object"));
-			configuration.properties = {};
-		}
-		for (let key in properties) {
-			const message = validateProperty(key);
-			if (message) {
-				delete properties[key];
-				extension.collector.warn(message);
-				continue;
-			}
-			const propertyConfiguration = properties[key];
-			if (!isObject(propertyConfiguration)) {
-				delete properties[key];
-				extension.collector.error(nls.localize('invalid.property', "configuration.properties property '{0}' must be an object", key));
-				continue;
-			}
-			if (propertyConfiguration.scope) {
-				if (propertyConfiguration.scope.toString() === 'application') {
-					propertyConfiguration.scope = ConfigurationScope.APPLICATION;
-				} else if (propertyConfiguration.scope.toString() === 'machine') {
-					propertyConfiguration.scope = ConfigurationScope.MACHINE;
-				} else if (propertyConfiguration.scope.toString() === 'resource') {
-					propertyConfiguration.scope = ConfigurationScope.RESOURCE;
-				} else if (propertyConfiguration.scope.toString() === 'machine-overridable') {
-					propertyConfiguration.scope = ConfigurationScope.MACHINE_OVERRIDABLE;
-				} else if (propertyConfiguration.scope.toString() === 'language-overridable') {
-					propertyConfiguration.scope = ConfigurationScope.LANGUAGE_OVERRIDABLE;
-				} else {
-					propertyConfiguration.scope = ConfigurationScope.WINDOW;
-				}
-			} else {
-				propertyConfiguration.scope = ConfigurationScope.WINDOW;
-			}
-		}
-	}
-	let subNodes = configuration.allOf;
-	if (subNodes) {
-		extension.collector.error(nls.localize('invalid.allOf', "'configuration.allOf' is deprecated and should no longer be used. Instead, pass multiple configuration sections as an array to the 'configuration' contribution point."));
-		for (let node of subNodes) {
-			validateProperties(node, extension);
-		}
-	}
-}
-
 const jsonRegistry = Registry.as<IJSONContributionRegistry>(JSONExtensions.JSONContribution);
 jsonRegistry.registerSchema('vscode://schemas/workspaceConfig', {
 	allowComments: true,