Sublime - duplicate multiple whole lines, not just selection

The duplicate_line shortcut in sublime, only duplicate the selections, when select across multiple lines.

How to duplicate the whole lines?

For example, I select last 3 chars of line 1, and whole line 2, then first 2 chars of line 3.
Aka. 3 lines selected, how to duplicate the whole 3 lines, not just the chars selected.

"Duplicate lines" only duplicates what is selected (unless nothing is selected, in which case it duplicates the whole line). If you have lines partially selected, first do Selection → Expand Selection to Line (CtrlL on Windows/Linux, ⌘L on Mac) to make sure the entire lines are selected, then do "Duplicate Lines".

Please note that if the last line is already fully selected and the cursor is on the beginning of the next line, that next line will also get selected. I advise you to play around with different kinds of selections to get a feel for how "Expand Selection to Line" works.