This error message occurs when you try to load an object using an object number which is already occupied. This can easily happen when using the load object command in loops. e.g
do
make object cube 1,100
loop
Will bring up an error because on the second iteration, it will try to create the object again but it already exists and so throws up an error.
make object cube 1,100
do
loop
works because the object is only created once.
An easy way around this is to use the object exist command
if object exist(2)=0 then load object "name.x", 2
Hope that helps