Corwin, Ur-Corwin, Corwin-bar, Norwic, Corwin of Avalon...

or

"Where did that extra Corwin come from anyway?"

The short answer first: we really don't know.

But that's no fun, so I'd make up a good story, except I've been beaten to it by someone else. Here's Keverian's theory:

A long, long time ago, Corwin and Eric had a fight. Eric won and left Corwin for dead in some deserted shadow. Corwin, however, is hard to kill and recovered from the encounter. Realizing that Eric thought him dead, he decided to take advantage of this fact: the sorceror King of Avalon, conjured an amnesiac duplicate of himself and left it on shadow earth to recover. He then retreated to his Shadow Avalon, which he made inaccessible.

Efforts to find Corwin met with failure until Flora discovered the duplicate on Shadow Earth. Flora, on behalf of Eric watches "Corwin." Meanwhile the real Corwin, plots and schemes unobserved and unhindered in Avalon.

Eventually, the shadow Corwin is found by others -- Bleys, Brand, Random -- and eventually makes his way back to Amber, where he he walks the Pattern, attempts and fails to sieze the throne, is imprisoned, escapes, is seduced by Dara, draws a second Pattern, and turns down the throne of Amber. You all know this story already.

Ur-Corwin -- the real one -- who has not undergone the grave personality changes so remarked upon by his relatives, siezes the gift given him by his shadow: a pattern all his own. He travels to Tir-na N'goth and there conjures a blue version of the Jewel of Judgement, which he uses to "edit" an images of Corwin' Pattern and gain control of it. Equipped with his own Pattern, he now begins to hatch his plans to make it the only order pole in the universe.

The only other interesting point worht noting about this whole theory is that this makes Keverian's father not quite the same person as Merlin's. Merlin would be the son of the shadow while Keverian is the son of the original. It's probably still safe to refer to them as brothers though. Certainly, Keverian considers that to be the case since he could walk the Pattern drawn by the duplicate.