Simulation versions getting over written

Hi,

I have created couple of simulation versions in planning board to check the feasibility of orders and have created in a different names.

But when I do a planning in plant, my simulation versions are getting over written by the active versions.

Means after that both my active real version and simulation verison looks same (simulation version data is replaced with active data).

Please let me know the reason and how we can restrict it.

Thanks,

Prasanth