the problem you are referring to is not trivial if you consider the symmetry of the new cell. This has to do with the maximal subgroups of a space groups listed at the end of each space group description in the International tables vol. A. For space group Fd-3m, there is no way to double the unit cell while keeping the same SG symmetry. You can double the unit cell e.g. by doubling one of the cubic lattice constant and end up with a space group with tetragonal symmetry. In this cas there are many possible space groups which can be derived on the basis of the different maximal subgroups 

Thank for your answer, but the tool appear not support more than 50 atoms per inputs, I get:

// ERROR ************************************************************************************

The total No. of atoms may not exceed 50.

Also the documentation is not so clear. I was reading and I'm not really sure whats options should be use to get new cell with double with atoms.

Can you explain how to get it?

Thanks! I get it.

But now I have one more doubt. If I use for example 2 unit cell in my calculation is like the same if I used 1 unit cell?

But is more precise?

The question if because I study doped materials.

Use p4vasp ... Is very good!  http://www.p4vasp.at/

the problem you are referring to is not trivial if you consider the symmetry of the new cell. This has to do with the maximal subgroups of a space groups listed at the end of each space group description in the International tables vol. A. For space group Fd-3m, there is no way to double the unit cell while keeping the same SG symmetry. You can double the unit cell e.g. by doubling one of the cubic lattice constant and end up with a space group with tetragonal symmetry. In this cas there are many possible space groups which can be derived on the basis of the different maximal subgroups 

I'm really sure that this not is very trivial problem. I used VESTA to generate other unit cell in z-direction and now I get 112 atoms. For one moment think that it was easy get the double quantity of atoms, But when I did some calculations in vesta the results aren't the expected.

For example I'm simulating LMn2O4 that one unit cell contain 56 atoms and 8 formula units, after double (2 unit cell and 16 formula units).

Also a attach an image: left is default unit cell and right is 2 unit cell.

You know other method that I can use for get the double of unit cell?

Thanks!