I have an assembly which is using several M3 x 20 cap head screws.  I cannot use Toolbox to insert another screw the same, I get the error:

You can see in the browser that the part is already used in the assembly.  I can copy the part in the assembly but I cannot now use this part in another assembly.

The Toolbox is installed 'stand-alone' on this machine.  I have seen this before and I can get round it by copying but i am losing the use of parts in the Toolbox.

Any ideas?