It's very useful feature, it should duplicate line with cursor or selected text when some assigned key is pressed
Product Version = NetBeans IDE 6.9 Beta (Build 201004200117)
Operating System = Linux version 18.104.22.168-0.1-desktop running on amd64
Java; VM; Vendor = 1.6.0_0
Runtime = OpenJDK 64-Bit Server VM 14.0-b16
It's already implemented. Ctrl+Shift+(Up|Down) in source code should handle that. Or via main menu - "Source" item.