By pressing CTRL+Z right after that operation, you remove those extra

elements (vertices, edges, or faces). But, in case you forget to press

CTRL+Z or use the Edit → Undo menu, all newly created elements will

stay present until you start to see visual problems from modifiers and

other operations.

You can easily fix that problem with a Merge using the distance option.

Look at Figure 3.25 to see a model that had a canceled extrude for one

of their faces.

Figure 3.25 - Model with duplicated vertices

Visually, the model doesn't seem to have anything wrong with the

polygon structure. But, if you select all vertices and press the M key

(Merge) and choose the "By Distance" option, you will see a message in

the status bar of Blender pointing that it removed four vertices (Figure

3.26).