As you can see, someone, most likely the developer, decided to package all the files into .rpa archives.
- Download Unren.
- Place the .bat file into the game folder (where the .rpa packages are located).
- Run the file and choose the first option to unpack the files.
- Then follow the steps mentioned in the main post.
The error message literally tells you that you have two gallery files, which is the problem:
"(...)
appeared twice, at
game/gallery.rpy:2102 and
game/system/gallery.rpy:2102"
If it's caused by the developer packaging the files, follow the steps above.