Handles multiple lines and leaves your cursor on the newly created end line. Accepts a direction that you want to duplicate your lines of text

duplicate_line.py

import sublime, sublime_plugin

class DuplicateLineCommand(sublime_plugin.TextCommand):
    def run(self, edit, direction='down'):
        self.view.run_command("expand_selection", {"to": "line"})
        view = self.view

        for region in self.view.sel():
            line = region
            if line.empty():
                line = self.view.full_line(region)
            line_contents = self.view.substr(line)
            if(direction=='down'):
                self.view.insert(edit, line.begin(), line_contents)
                newline = sublime.Region(line.b, line.b+line.size()-1)
            else:
                self.view.insert(edit, line.end(), line_contents)
                newline = sublime.Region(line.a, line.a+line.size()-1)
            self.view.sel().clear()
            self.view.sel().add(newline)