same author, same year - add letters a,b,...


I am currently using this style:

If I have different sources which are from the same author published in the same year, I want it to add a,b,c.. after the year. In the footnotes as well as in the bibliography.

Smith, P. (2001a)
Smith, P. (2001b)

How can I edit the code for that to happen?
Thanks a lot for your help!
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