You can't just do a straight copy of the VDI image of your virtual machine because there is a unique UUID, wellI believe you can and then you can create a new UUID manually (at least you could with VMWARE) but to keep it clean just use this method:
VBoxManage clonevdi /path/to/your.vdi outputname.vdi
0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100%
That's all there is to it, just wait for it copy and you can th........