"yes it can be part of a SIP DIP and/or AIP"
"no there doesn't have to be separate copies."
The OAIS is mostly about defining terminology. Where it is silent I'd say it is up to us to interpret.
I personally see SIPs, DIPs and AIPs as conceptual. It is conceivable that a digital preservation system workflow could include putting a file in a folder, having a SIP created that references that file, having an AIP created that references that file and other backup copies that are made during the process, and having a DIP include the same file stored in that same original location as the mechanism for accessing that object.