To recreate:

1) Create a template from a document
2) File -> templates -> edit and choose template created in 1
3) Edit the template.
4) File -> templates -> save & choose the same file name as template created in 1
5) Answer "Yes" to the name already in use, overwrite document template dialog.

Now presented with 2 error dialogs the first claiming there is an error saving
the file as it already exists and the 2nd saying "Error writing document
"docName" as template: Access to ... was denied."

A new file is created with a number appended to the end of the file name. 

In the list of templates to create a new document from the duplicated templates
aren't shown, but when trying to edit a template again you are given a list of
all the templates.