Changing text applied when duplicating file in Finder

1. Open /System/Library/CoreServices/Finder.app/Contents/Resources/English.lproj/

   ^{Use Show Package Contents to browse the package in Finder, or open using Terminal, etc.}

2. Open Localizable.strings in BBEdit or other editor capable of editing .strings files.

3. N4 contains the string that is used to name the duplicate file.

   ^0 copy
   

   ^{^0 is the previous file's name. Make sure to keep this to retain the name of the previous file.}

4. Edit the string how you wish. For example, to duplicate test as test_copy instead of test copy, use…

   ^0_copy
   

5. Save and relaunch Finder.

   killall -HUP Finder
   

2020: macOS Catalina Version

As mentioned above this is a hack and will disappear with the next update you do on your mac.  

1. Create a copy of the file specifying the Finder language specific texts. i.e. to the Desktop

cp /System/Library/CoreServices/Finder.app/Contents/Resources/en.lproj/Localizable.strings ~/Desktop/

2. Open with BBEdit

bbedit ~/Desktop/Localizable.strings

3. Search for <key>N4_V1</key> and replace <string>^=1 copy</string> as desired and described above and save the file.

    

4. Disable rootless as shown here.

   • Reboot with Command + R keys into Recovery Mode
   • open a Terminal, disable rootless and restart

csrutil disable; reboot

5. After reboot, make your Macintosh HD writeable as described here

sudo mount -uw /
killall Finder

6. Copy your change file to the original location and restart Finder as mentioned above:

sudo cp ~/Desktop/Localizable.strings /System/Library/CoreServices/Finder.app/Contents/Resources/en.lproj/Localizable.strings
killall -HUP Finder

7. Enable rootless as shown here.
   • Reboot with Command + R keys into Recovery Mode
   • open a Terminal, enable rootless and restart

csrutil enable; reboot

Enjoy a simplified workflow :-)

Option + drag a file into the same directory.

Or option + drag a file into the directory that contains a file with the same name and select keep both.

These actions will append a number "2" without the word copy. If a file with a "2" exists the Finder will append a "3" instead!

You could also multiple-select and then Option + drag into the same directory and the numbers will all increment nicely. It's beautiful.