Duplicate keys in dict?

[vsoler]

Hello,

My code snippet reads data from excel ranges. First row and first
column are column headers and row headers respectively. After reding
the range I build a dict.

................'A'..............'B'
'ab'............3................5
'cd'............7................2
'cd'............9................1
'ac'............7................2

d={('ab','A'): 3, ('ab','B'): 5, ('cd','A'): 7, ...

However, as you can see there are two rows that start with 'cd', and
dicts, AFAIK do not accept duplicates.

What is the best workaround for this? Should I discard dicts? Should I
somehow have under 'cd'... a list of values?

One of the difficulties I find here is that I want to be able to
easily sum all the values for each row key:  'ab', 'cd' and 'ac'.
However, using lists inside dicts makes it a difficult issue for me.

What is the best approach for this problem? Can anybody help?