Thanks for the nice reply. But here I found in my system, if I assign the existing plant respect to the other Business Area, that is not allowing me to save. Like respect to the above screen shot, if I select Plant ID "0001" respect to Business Area "ST00", that's giving me an "Entry already exits with the same key". However, I selected the values in my system by using F4 (Existing Values only). Just for discussion purpose I given your example.
In my case, same plant/plants can be present in multiple Business Unit.
Please advise me for the same.