You can't just do a straight copy of the VDI image of your virtual machine because there is a unique UUID, well I 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 then create a new VM with your cloned VDI.  This is nice if you want to create templates etc... or if you want to make a copy of a testing machine for more testing.