createProjectFromXML is not supposed to be called for a project which already...

createProjectFromXML is not supposed to be called for a project which already exists, but failed to enforce this.

core/src/main/java/hudson/model/ItemGroupMixIn.java

@@ -223,6 +223,10 @@ public abstract class ItemGroupMixIn {
         acl.checkPermission(Item.CREATE);
 
         Jenkins.getInstance().getProjectNamingStrategy().checkName(name);
+        if (parent.getItem(name) != null) {
+            throw new IllegalArgumentException(parent.getDisplayName() + " already contains an item '" + name + "'");
+        }
+
         // place it as config.xml
         File configXml = Items.getConfigFile(getRootDirFor(name)).getFile();
         configXml.getParentFile().mkdirs();