Trying to rename repeated columns in pandas DataFrame

Question

I have a list of duplicate columns in my DataFrame (named as df here in the code) and I am trying to rename duplicate columns by adding a number in the end. For example:

If there are two columns named col, then I want two columns named col1 and col2.

But this code gives two column names as col1 and col1 again.

for col in duplicate_col:
    counter = 1 
    for col2 in df.columns:
        if(col2==col):
        df.rename(columns={col2:col2+str(counter)},inplace=True)
        counter = counter+1