For one reason or another, it always happens, the need to rerender. Perhaps a small tweak to a keyframe or a timing issue. Chances are while your animation may have needed refining, the render queue was perfect. Save yourself some time and Re-Queue that animation.
Select a previously rendered animation.
Press Command + Shift + D (Control + Shift + D) to duplicate the render item with the same settings and output name . This will rewrite over the previous file to the exact same destination.
Save your work, then click Render when ready to output.