Collections.Counter in Python3
I have no idea what you code is doing, it doesn't look like any normalization I've seen. I'd offer a more specific suggestion on restructuring if I understood what you are doing.

You:


Put a in a counter
Put that counter's values into a set
sort the keys of the counter


I'd look for another approach that doesn't involve so much moving around.

if not previousCount == count:


better as

if previousCount != count:


EDIT


Counter.most_common returns what you are fetching using sorted(c, reverse=True)
itertools.groupby allows you group together common elements nicely (such as the same count)
enumerate can be used to count over the elements in a list rather then keeping track of the counter


My code: 

c = Counter(a)
length = len(set(c.values()))
counter_groups = itertools.groupby(c.most_common(), key = lambda x: x[1]))
normalisedValueCount = {}
for i, (group_key, items) in enumerate(counter_groups)
    for key, count in items:
        normalisedValueCount[key] = (i+1)/length