To duplicate lines I use
Ctrl + D
Is there a similar way to select/copy current line using keyboard in IntelliJ IDEA?
If you have nothing selected, I think you can just hit CTRL-C to copy a line (or CTRL-X to cut it)...
http://grahamhackingscala.blogspot.co.uk/2010/08/fast-quick-line-cut-copy-intellij-idea.html
Hope this helps.