When I Duplicate it adds " copy-1" to every item

I duplicated a folder with more than twenty interior documents, and each item had “copy-1” added to it’s name.

Is there a way to prevent that or “batch change” them so I don’t have to go back and manually correct each name?

The interior documents shouldn’t have “copy-1” added to their names - only items in the top-level of the copy get “copy” appended to their names. (You can use Simple Duplicate to duplicate without children or altering the name.)
All the best,
Keith

Oh! So if I duplicate the parent folder, rather than all (or some, as I was trying to do) of the interior file, then the interior ones won’t have the “copy-1”. That makes sense.

I thought I’d done it before without that happening, I must have copied a whole folder. Great, thanks!

Yep, that’s the idea. Or if you only want to copy a few of the interior files, use “Simple Duplicate” instead. “Duplicate” and “Simple Duplicate” have been renamed to make their functions clearer for 2.0. They are both placed in a “Duplicate” submenu and have become Duplicate > with Subdocuments and Unique Title (cmd-D - the same as current Duplicate) and Duplicate > without Subdocuments (shift-cmd-d - the same as current Simple Duplicate). Your post has presented a minor bug, though - obviously files that are copied should become “[filename] copy”, and then the second file with the same name should be “[filename] copy-1”, as in the Finder, so I have fixed this minor glitch for 2.0.

Thanks and all the best,
Keith