Complex sentence: Some historians identify him with Peter of Diokleia, whose seal was found in the 19th century, so he could be the earliest Duklja's ruler whose existence was confirmed by primary historical sources.
Split into simpler sentences: Some historians identify him with Peter of Diokleia, whose seal was found in the 19th century. He could thus be the earliest ruler of Duklja whose existence was confirmed by primary historical sources.