Use a list instead of a string:
FinalData = {'12345,70': ['xyz'], '12345,71': ['pqr']}
and this works:
FinalData[key].append(value)
Example
key = '12345,70'
value = 'abc'
FinalData[key].append(value)
print(FinalData)
Output:
{'12345,70': ['xyz', 'abc'], '12345,71': ['pqr']}
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