Duplicate line or selection
One of the ways of creating a code element similar to an existing one (for example, a new method overload or a new auto-implemented property) is cloning the existing element with copy-paste and then editing the clone.
JetBrains Rider helps you streamline the cloning part — with a single keystroke Ctrl+D the cloned code is inserted right after the original.
Duplicate the current line or selection
If you want to clone a line, set the caret at this line line. To clone an arbitrary piece of code, select it in the editor.
To select a logical code block, press Ctrl+W one or more times; to select the current declaration, press Ctrl+Alt+Shift+[.
Press Ctrl+D or choose from the main menu. Alternatively, you can press Ctrl+Shift+A, start typing the command name in the popup, and then choose it there.
The duplicated line or multi-line selection is inserted below the original line or selection; the duplicated inline selection is inserted to the right of the original.
If you have duplicated a selection, the selection stays at the duplicate. If necessary, you can press Ctrl+Shift+Alt and then use arrow keys to move the duplicate to the desired position.
Note that when you duplicate a selection (even if the selection spans multiple lines), the duplicate is inserted right after the initial selection, without a line break.
If you want the selection to be copied as whole lines and pasted starting with a new line, use the Duplicate Entire Lines action.
This action doesn't have a keyboard shortcut by default, so you can either use Go to Action Ctrl+Shift+A to invoke it, or add a keyboard shortcut to it.