TaxonomyPlugin: Same key in tree causes NPE

Reproduction path:

• Start up the taxonomy demo project
• Delete the existing taxonomy tree under '/content/taxonomies/exampletaxonomy'
• Import the 'taxonomytree.xml' attachment under '/content/taxonomies'
• edit the document '/content/documents/taxonomydemo/common/homepage' and click on the 'Select taxonomy terms' link
• navigate to 'exampletaxonomy/somecategory/anothercategory' and click on the triangle next to 'anothercategory' to collapse the category

Expected result:
The category collapses and its children are shown.

Actual result:
A NullPointerException is thrown due to there being two same keys in the tree.

Could this issue be related to HIPPLUG-159?