[Python] Clear value on a specific column using openpyxl

I have 2 questions. And I want to solve do it on openpyxl since I already formatted my excel file and want to do the following.

1. 
   Remove all the duplicates only remain the first value
   
   
2. 
   Remove all the value on the Column A that have "Subtotal" in its value.
   

For reference here is my data set

Column_A Column_B Column_C
1 A 10
1 None 11
1 None 12
1_Subtotal None 33
1_Subtotal_1 None 33


Expected Outcome:

Column_A Column_B Column_C
1 A 10
None None 11
None None 12
None None 33
None None 33


I tried using this code for removing the subtotal

for cell in ws['Column_A']:
if 'Subtotal' in str(cell.value):
cell.value = None


But it won't work. Thanks!

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