"Doclea" the name of the region during the early period of the Roman Empire, was termed for an early Illyrian tribe. In later centuries, Romans "hyper-corrected" to "Dioclea" wrongly guessing that an "I" had been lost due to vulgar speech patterns. "Duklja" is the later Slavic version of that word.
It was one of the four southern Dalmatian Slavic duchies, other three being Narenta (Pagania), Zahumlje and Travunia that weren't united under either Croatia to the northwest or Rascia (Serbia) to the northeast.
One of the famous mentions of Duklja is the Chronicle of the Priest of Duklja which was written by a priest from Dioclea (Bar) around 1180-1196 and which includes various information about these and related territories.
Duklja was populated by Christians of the Latin Rite and was initially usually under the suzerainty of the Byzantium or of Bulgaria up to the 11th century when the dukes of Duklja started having noticeable success in their struggle for independence.
Starting in 1036, Dobroslav, also called Stephen Vojislav, who was a descendent of both Serbian (from Travunia) and Croatian royal families, liberated Duklja from the Bulgarians for a short period. Later his achievements were repeated by his descendents Mihail, Bodin, Vukan, Marko, Uroš I, Uroš II and finally Stefan Nemanja who later expanded Raška to include Duklja and other Catholic territories and became the founder of the Nemanjić Serbian royal dynasty, the first dynasty to have a member of it recognized by the Pope (Stefan Prvovenčani in 1217).
Nemanja himself was initially baptized under Roman rite like the other rulers of Duklja, and later rebaptized under Byzantine Rite. He gave the rule over Duklja to his son Vukan Nemanjić, and due to the difference in religions the two ostensibly united territories were at odds. Eventually though the Serbian influence prevailed.
The relationship between the regions of Duklja and Zeta is somewhat unclear. Duklja was mostly referenced as the littoral area between the Bay of Kotor and the Skadar Lake, while Zeta refers to the river located inland and is thus the more accurate predecessor of the 19th century Montenegro. According to another interpretation, Duklja was composed of Zeta and Travunja (Travunja was roughly today's eastern Herzegovina and Konavle ). In any event, the name "Duklja" went out of use by the end of the Middle Ages.