I cant copy and paste (any mode)

I used copy and paste (CTRL C and CTRL V) yesterday for a whole day, I was making a piano.
Today, I used it too, when building a house, but all in a sudden it doesnt work.
Its like I was using CTRL X and CTRL V, but I am not.

Altough Shift D works but thats not the same.

At right in the bar with all objects, the object copies and is twice there, but its like there were two cubes in one. Both is the same cube.

Does anyone know how to help?
Thanks a lot,