The first events we know about in Duo's life occured in AC 187, when he was already seven years old. He appeared on the V08744 at L2, which was very poor due to the fact that most of the its residents were impovrished immigrants from all over the Earth. Because of the wide cultural spectrum present, the colony was poorly organized, restless, and quickly became anti-federation. There was a great deal of fighting in the colony, which left behind a lot of abandoned buildings, as well as orphans.
Duo was not born on the colony; rather, he drifted there, probably by stowing away on a shuttle. It's quite possible that he, too, was an orphan. At that time, he didn't have a name, which is another fact that points towards him being an orphan. He was taken in by a kid named Solo, who led a group of orphans.
Solo was infected with a virus that was burning its way through the entire colony. A vaccine had been discovered, but the authorities saw no reason to distribute it to the poor or homeless. Duo snuck into a hospital and stole enough vaccine for everyone but himself...but it was too late for Solo. Before he died, Solo said to Duo "You're a nice guy...I wish I could stay with longer." Duo replied, "Don't be stupid...you shouldn't give up." Later, Solo said "Even after I die, I'll always be with you...so be nice." When Solo died, Duo said, "Fine...if Solo's with me, we'll be Duo." (Yes! We have a name!)
When Duo realized that he didn't have any vaccine for himself, he decided that he didn't mind if he died. He thought, "I'll die too...because we're Duo." He didn't even get sick, though. Perhaps Solo really was watching over him. He decided to stay on as leader of the group in Solo's place.
Maxwell Church
One day, perhaps out of anger at the Federation, he took the orphans to a Federation base to steal food. They were caught and managed to escape, but the people of the colony tore down the abandoned building they had been living in. Maxwell Church took all the orphans in. They fed them and gave them clothes (like the ones that Duo still wears when he's a pilot), and one of the nuns, Sister Helen, tried to cut Duo's long hair, but Duo refused to let her. So instead, she braided his hair, just for the sake of neatness, and it seems that Duo came to like the hair style. He said "Now it won't get in the way when I steal stuff."
School Days
Duo and the other orphans were sent to a Federation-run school, which was normally for children of military personnel. The school accepted the kids for the purposes of brainwashing. (Lovely)
Duo constantly got into fights with the military brats, who looked down on the orphans. In spite of that, he was the top student in his class, which really only made matters worse for him, because he didn't have to study; he learned extremely quickly. The orphans that Duo had been with were adopted out, but Duo never managed to find a permanent family; he always got kicked out of the house in a day because he was such a trouble maker.
One day, Duo got into a very bad fight with a group of five kids; he ended up putting them all in the hospital. Because of this, he was threatened with expulsion from school. When Sister Helen asked Duo why he had gotten into a fight like that, he said it was because the kids had called him names and told him he smelled like a sewer. (Water was very scarce in the colony, and the poor people barely had enough to drink. If they were lucky, they could wash their clothes once a month.) Sister Helen hugged Duo and told him that he didn't smell, and that they would collect some snow for water so that they could wash. They got snow from off the roof of the church. Father Maxwell came back to find them; he'd just finished apologizing at the school for Duo. Duo felt really bad, but Father Maxwell said "It's alright. If they only want me to apologize for you, I'm happy to do so. Just go back to school tomorrow and work hard."
Father Maxwell and Sister Helen were Duo's first real family.
Massacre
The next year, there was another uprising, turning the colony into a battlefield once more. Maxwell church became a makeshift hospital for the wounded for the resistance fighters that had made it their base. One of the resistance leaders decided that they needed an MS (mobile suit). When Father Maxwell objected, bringing up Heero Yuy (the colony leader, not the Gundam Pilot), they struck him with the butt of a rifle. Also, they accused Duo of being a Federation spy, since he went to school on the base. He told them that he would steal an MS for them if they would just leave the church, since the church was a place of sanctuary. The didn't believe him. All he had to say to that was, "I may run, and I may hide, but I don't lie."
Duo ran to the Federation base. He knew the layout very well this time, so that he wouldn't get caught like he had when he was trying to steal food. He had to run through gunfire, but he managed to make it to a truck that had a Leo in it, and he drove it back to the church.
When he arrived, the church had been destroyed. He found Sister Helen in the ruins. She said "Duo, I'm glad you're safe. The Father was worried about you until the end..." Duo was devastated, but he refused to cry, instead, smiling for her. He tried to leave so he could get a doctor, but Sister Helen kept talking. She said "The Federation forces attacked us, but we couldn't leave this place." Duo replied, "It's my fault. I stole the Mobile Suit." Sister Helen was now dying; she was having a hard time focusing. "The Father urged peace until the end." She touched Duo's cheek very gently and said, "May God protect you..." before her hand fell away.
Duo cried and screamed like a lost soul, because he would never hear her sing again, or feel her warmth. It was the first time he had ever cried because of death.
The incident later became known as the "Maxwell Church Tragedy." 245 people died. It is unknown where Duo went after this; he disappeared and didn't resurface for several years.
The Gundam Pilot
No one is quite certain when Duo hooked up with Doctor G, and the circumstances of their meeting are somewhat unclear. Duo was a stow-away on a ship that Doctor G was on. He was caught and dragged up onto deck. Doctor G must have seen something that he liked in Duo; he not only allowed the boy to remain on the ship, but he welcomed Duo into the group, and eventually gave him Deathscythe to pilot. Duo, notably, was not happy when he found out the true purpose of the Gundam. He set charges and attempted to blow Deathscythe up. Doctor G caught on, however, and disarmed the bombs before Duo attempted to detonate them. He then confronted Duo and told the young pilot to steal Deathscythe and go to Earth to fulfill the mission as Duo saw it.