does the ancient mariner have a doppelganger ?

The mariner speaks of having a doppelganger, though he means it figuratively, in the following lines:

Like one, that on a lonesome road,

Doth walk in fear and dread,

And having turned round walks on,

And turns no more his head,

Because he knows, a frightful fiend

Doth close behind him tread.

He feels haunted by a curse, something evil that is always on his back. But because he caused his own curse by killing the albatross, the unfortunate figure is himself. It's a classic doppelganger, a version of oneself that represents the darkness. In this case, that other side haunts the mariner.