PluginManager's typeList shouldn't contain duplicates

I'm using the Digibiz Dictionary Section (http://our.umbraco.org/projects/backoffice-extensions/digibiz-dictionary-section) in a v6.1.3 project, but the Tree is showing up twice in the UI.
By debugging I found out the type is added twice, I couldn't figure out why though...
Adding this line prevents the tree showing up twice.

(duplicate of https://github.com/umbraco/Umbraco-CMS/pull/71)