Sometimes, things happen when you don’t want them to happen. In the case of Blender, they are a terrible headache to correct, even when you have the wondrous Ctrl + Z. One of those awful mistakes is when two individual vertices are at the same point on your model! How something cannot be seen could cause so many inconveniences? Don’t worry about that anymore! You will learn how to correct double vertices in Blender.
When you use the Extrude Tool, you must be careful because the Ctrl + Z just undo the movement part of the operation but not the extrude itself. That means the vertex created for this function remains even if you undo the last action and cannot see the other faces made.
Tip: if you want to know if you have a double vertex, you can select one and move it out. If you see that the vertex moves independently and leaves no trace of another, you only have one. But if, when dragging, you observe another vertex at the starting point, then you need to fix it.
You can solve and clean this up by merging any duplicate vertices you have; you just need to select both vertices, press M on your keyboard to bring out the Merge Menu, and choose the option that fits you best.
If you have an active selection (you can recognize it by the green highlight), you can also select “At last” or “At first” to move the vertex to one of those points.
On the other hand, selecting the “By Distance” option on the Merge Menu will help merge many overlapped and duplicate vertices that are difficult to see. To solve that, select the whole model and click this option; the program will tell you how many vertices have been removed.
There you have it. No more nano-problems you cannot see. Good luck!