Copy command works on the same basis as move. ie. Select objects push enter and follow the command line instructoins.
Specify base point means "Copy these selected objects from this (Specify-left click) point to this new (Specify-left click point)
Note that copy automatically provides multiple copies until you push enter to finish the copy command.
If you continue to create multiple copies you can just left click points where each multiple copy will go.
If you are copying objects at set distances the second and subsequent distances will be based on the original base point and therefore accumulative. ie. 600,1200, 1800,2400 etc.
If there are more than 2 copies required consider using the rectangular array command which is designed to produce multiple copies in rows and columns at specified distances apart.