How to select every second object?
Okay well if you want a simple way to batch delete duplicates with the naming convention photo.file, photo(1).file you can do it very easily from the terminal.

First open up your terminal. Then navigate to the folder you are trying to delete stuff from. This is done by using the cd command. 

For example, if your photos are stored in your_username/Desktop/my_photo_album you would type cd ~/Desktop/my_photo_album. 

From there you can type ls to make sure you are in the right folder, and it should list the contents of your folder. After that, all you have to do is type:

rm *"(1)"* and it will delete any photo that has (1) in the name!