Command entry: Track View
Keys Toolbar
Move Keys flyout 
(Move Keys),
(Move Keys Horizontal), or
(Move Keys Vertical)
Command entry: Track View
Curve Editor
Key Controls Toolbar
Move Keys flyout 
(Move Keys),
(Move Keys Horizontal), or
(Move Keys Vertical)
You can use the the Move Keys tools in the Curve Editor to change animation keys' timing, values, or both. The same tools are available in Dope Sheet mode, but they all have the same effect: changing keys' timing (horizontal positions) only.
You can clone keys by holding down Shift while dragging. New keys are added to the existing keys in the same track.
To move a selection of keys in Curve Editor:
Only keys in highlighted tracks appear in the Key window.
By default, the Move Keys button is highlighted, showing it is already on.
If you drag a highlighted key on top of another key, the moved key replaces the other one.
To move a selection of keys in Dope Sheet:
(Move Keys) if it isn't already highlighted. (It should be on by default.)
The Move Keys tools are available from a flyout on the Keys toolbar in both Track View modes, and also on the Key Controls toolbar in Track View - Curve Editor. The default tool, Move Keys, lets you move keys in any direction in Curve Editor. In Dope Sheet, all three tools allow horizontal (timing) movement only.