Dupliate AST nodes in Python

[fwendland]

I've identified the following test case resources, where duplicate AST nodes occur:

• src/test/resources/python/issue615.py
  Reference[name=loopMultiVarHelperVar_ffffffff-ea1a-9ec7-0000-000000fc5b65,location=issue615.py(2:5-2:15),type=UNKNOWN,refersTo=Variable[name=loopMultiVarHelperVar_ffffffff-ea1a-9ec7-0000-000000fc5b65,location=issue615.py(2:5-2:15),initializer=<null>,modifiers=[]]] -> 2
• src/test/resources/python/1740_endless_cdg_loop.py
  Reference[name=loopMultiVarHelperVar_ffffffff-ea1a-9ec7-0000-0000287dfac6,location=1740_endless_cdg_loop.py(3:9-3:19),type=UNKNOWN,refersTo=Variable[name=loopMultiVarHelperVar_ffffffff-ea1a-9ec7-0000-0000287dfac6,location=1740_endless_cdg_loop.py(3:9-3:19),initializer=<null>,modifiers=[]]] -> 2

Initial analysis:

• The issue seems to occur with loops using tuple unpacking, where a helper variable is reused.