Mango
Jaya & Vijaya were once cursed by Sankadik muni for obstructing them from seeing Vishnu without his approval. THe curse was that they'd either be born 3 times as powerful enemies of Vishnu to be killed by him or several times as great devotees of Vishnu. Since death at the hands of Vishnu would liberate them from subsequent rebirth cycles once the t&c of the curse had been fulfilled, and since they wanted to get back to Vaikunta sooner rather than later, they opted for being born Vishnu's enemies. In the Krita yuga, they were born as Hiranyaksha and Hiranyakashipu (killed by Vahara and Narasimha), in the treta yuga as Ravan and Kumbhakarna, and in the dwapar yuga as Kamsa & Shishupala (some accounts have it as Shishupala and Dantavakra, although the latter was a very insignificant warrior, in contrast to Kamsa, who was quite powerful even without the backing of Jarasandha).
One thing - from what I've read, none of these incarnations had any idea that they were Jaya & Vijaya.