In Windows using the command prompt you can delete all files including [1] in their name by using:

DEL /F /S /Q /A "C:\Users\USERNAME\Documents\*[1]*"

Replace the /Q with /P if you want to be asked for confirmation on each file

Be careful that you don’t delete files you didn’t mean to – but in the example I was working on, multiple copies of each file were restored from a corrupted hard drive and there were copies of filename.jpg as filename[1].jpg and filename[2].jpg, so the above command a couple of times deleted everything that was duplicated perfectly