The Nitrogen is now attached to the 2 position of the chain; its lone pair again attacks the carbon of the other carbonyl, joining the chain, and again protonating the oxygen and deprotonating the nitrogen.
The Nitrogen is now attached to the 2 position of the chain. Its lone pair again attacks the carbon of the other carbonyl. Its lone pair again is joining the chain. Its lone pair again is again protonating the oxygen. Its lone pair again is deprotonating the nitrogen.