I have a question we are creating two files in aiml first is *.aiml and another is *.aiml.csv.
I am not able to understand the logic behind it why we need two files. However, as I checked in program AB, if I modify the code only in *.aiml.csv file. then Bot will give response according to that code in *.aiml.csv file.

Moreover, as per my knowledge we always avoid code duplication and here we are doing same. So, I am confused and want to understand the concept behind it. I really thankful if you explain it.

I don’t think you have to make two files at all. Those are two options which are available. If you were to make simply an AIML file or a CSV it would work fine.
I’ve found you have to delete all the CSV files first though, before you run the AIML files. And then it works fine.
If you look at this and this you’ll see that it is an option.

The CSV files are a form of “compiled” AIML that are created automatically when the AIML is first loaded.

Their purpose is to provide a faster load time than the raw AIML files.  They can also be used if you want to edit the AIML in a spreadsheet like Excel or Google Sheets.

You can delete all the CSV files, and they will be re-created automatically when the AIML is loaded again.  I tried to build in some logic to make them refresh whenever one of the AIML files changes (based on the file creation dates) but it’s been a while since I tested it.