Are the Greeks the original victims of Plagiarism?

It seems that yet again, Dante's The Divine Comedy is ripping ideas off of the original Greek work, The Odyssey.  We first saw the Greeks being copied when the Romans copied off of them as Virgil wrote the Aeneid.  Virgil was asked by Augustus to write the Aeneid to make Roman society seem sophisticated like Greek society was.  Now again, it seems that Dante is plagiarizing many ideas off of both The Aeneid and Odyssey as he takes a "journey through Hell."  It is interesting that works such as the Aeneid and The Divine Comedy are so famous when they repeat many of the same ideas from previous literary works.