Changing text applied when duplicating file in Finder
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.
Open Localizable.strings in BBEdit or other editor capable of editing .strings files.
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.
Edit the string how you wish. For example, to duplicate test as test_copy instead of test copy, use…

^0_copy

Save and relaunch Finder.

killall -HUP Finder