There is an Adhika Masa or Adhik Masam in 2020 from September 18 to October 16 and here are the general rules followed in Karnataka and Andhra Pradesh regarding performing Shradha, Tarpan and other funeral rituals. In Kannada and Telugu calendars, the extra month is the Adhik Bhadrapad month.
The yearly Shraddha occurring in
Ashwin Masa should be only performed in the Nija Ashwin Masam not in the Adhik month.
The first yearly Shraddham of the person who died in the
Bhadrapad month last year should be performed in the Adhika Ashwin Masa.
The monthly Shradha of those expired in Ashwin month should be performed in both Adhik and Nija Ashwin month.
If the 12th Masika occurs in Adhika, then after performing 12th Masika in Adhika Ashwin and in Nija Ashwin, Prathama Abdika should be performed in the14th Month.
Darsha Shraddha, Nitya Shraddha, Yugadi-Manvadi Shraddha should be performed in both Adhika and Nija Ashwin Masa.
Prati-Samvatsarika Shraddha of those who expired in last Adhika Masa should be performed in Adhika Ashwin only. It should not be performed in Nija Ashwin.
Please note that this information is as found in various South Indian Panchangas which has Adhik Masa.
This post is only meant to give a general idea. You should also take expert advice from priests or genuine astrologers.