This plugin for the gedit text editor allows to duplicate lines, just like in Geany, where you can press CTRL+d to duplicate either the current line or the current selection. It is compatible with Gedit >=3.14.
Drop the files contained in this package into your ${HOME}/.local/share/gedit/plugins/ folder. If the plugins folder does not exist yet, just create it.
Start gedit and open Preferences and in the Plugins section activate the Duplicate Line plugin.
The plugin adds the keyboard shortcut CTRL+SHIFT+d to gedit. When pressing this key combination the plugin will duplicate the current line. If there is a selection and the selection spans several lines, then all selected lines are duplicated. If the selection is within a single line, then the selected text will be duplicated.