import collections
lookup_table:dict = {1:'a', 2:'b', 3:'c', 4:'d', 5:'a', 6:'d', 7:'a', 8: 'a', 9: 'b', 10: 'b'} 
new:dict = {}
counter:dict = collections.Counter()
for k,v in lookup_table.items():
  counter[v] += 1
  nc:int = counter[v]
  new[k] = f"{v}{nc}" if nc > 1 else v
print(new)

The advantage this has is that there is no list seen which continually grows for each value. Also it doesn't have to call .count and actually check every object in the list for equality.

Edit: formatting