The error means that the rendered DropDownList object has two items with
selected="true" which is not allowed. The template selector is the only dropdown I can find in that page. The template selector items are generated automatically depending on your controllers. If this issue happens for all page types then it's likely to be a case of duplicate default templates being registered. Assuming you're using MVC, look for Controllers that are using the
[TemplateDescriptor(Inherited = true)] attribute in your code. Then if you have multiple of them try renaming one using the
Name="myname" parameter of the TemplateDescriptor attribute.
If you want to dive deeper you can use something like the
ITemplateRepository service to get all the registered templates and inspect them for duplicates.
For reference this is what the template selector looks like:
Hope this helps
We get this error when trying to access page setting from within the Admin area.
Error "Cannot have multiple items selected in a DropDownList" after clicking the Setting button on the Admin->Content Type->Page Types-Settings
Cannot have multiple items selected in a DropDownList.