Need Help! Unable to use Grab correctly.

[Yumilong]

I select two vertices and press the "G". However, Blender create some new vertices instead of moving the selected ones. I feel very confused.
PS， I am in the edit mode and the grab tools works fine before.

[MatejMo]

...the grab tools works fine before.

Did you mess with key settings?

[Yumilong]

...the grab tools works fine before.

Did you mess with key settings?

I think it must be some key i press incidently.
So, I save my project and create a new file using loading the factory setting.
Then, everything works fine. I can use grab tool property.
But as if i reload my project, select vertices and press "g", grab tools still unable to work in the right way.

[Tehrasha]

At some point you may have extruded the verticies but did not move them.
So when you selected them to move them, you only grabbed one pair.

'Remove Doubles' would have also fixed. it.