Transformation of object vertices from the Standard Asset. Problem.

For example, I have a house where I use a piece of wall with a window opening from the Standard Asset. I put a new piece of wall on top of the wall from a similar piece to the first one, but I need to narrow that piece of wall down a bit. I edit (move) vertices of the side of this piece of wall and… after pressing Accept, I see that the first piece of wall has exactly the same vertices shift! Why is the second piece suddenly somehow attached to the first? And how do I turn it off, if it’s possible?