Fix duplicate angle in dictionary error

[Python1320]

The commit https://github.com/Metapyziks/VMFInstanceInserter/commit/9487796dbda67c12e4c8e6508b980125b82a1db2#diff-b6cd5396d3ebfa4934fa608dc1f60ea4R179 introduced the following error:

Unhandled Exception: System.ArgumentException: An item with the same key has already been added.
   at System.ThrowHelper.ThrowArgumentException(ExceptionResource resource)
   at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
   at VMFInstanceInserter.VMFStructure.ParseFGD(String path)
   at VMFInstanceInserter.VMFStructure.ParseFGD(String path)
   at VMFInstanceInserter.Program.Main(String[] args)

I have not tested this change much, but it should fix it.