Deal with resources with same name

[philbarker]

1. start a new session
2. open RDFS file, select sdordfs.ttl

Result: error, "..._tkinter.TclError: Item http://schema.org/TherapeuticProcedure already exists" (or similar); import of RDFS is halted

Expected Result: should cope with existing resource info without halting import

[philbarker]

Info: the error is due to trying to insert two items in class tree with same id, b/c class id is used as id in tree and same class can occur in different branches of tree. Perhaps add an index number to class id before using as tree item id. Need to make sure that can still use item id in tree to identify class in graph.