What you wan't to achieve is technically not feasible. Key to a dictionary cannot be duplicate because if so you cannot uniquely index a dictionary with a key.
What you can do, is to create a dictionary of (key,value) pair where value is the list of all items which has the same key. To achieve it you can do something as follows
>>> person_year={}
>>> [person_year.setdefault(v,[]).append(k) for (k,v) in year_person.iteritems()]
[None, None, None, None, None, None, None]
>>> person_year
{'Bruce': [2002, 2004], 'Linda': [2000, 2003, 2006], 'Ron': [2001], 'Gary': [2005]}
>>>
Note, if you are only interested in the key value pair and not a dictionary per se' you can just store as a list of tuples as follows
>>> [(v,k) for k,v in year_person.iteritems()]
[('Linda', 2000), ('Ron', 2001), ('Bruce', 2002), ('Linda', 2003), ('Bruce', 2004), ('Gary', 2005), ('Linda', 2006)]
>>>
