They shared a phone, a car, clothes, a Facebook profile and lived in the same apartment in Marseille.
But what they did not share was the urge to go out and rape young women.
But, because they have almost identical DNA, it took police months to figure out which of the twins committed the crimes.
The alleged rapist, 26-year-old Yoan Gomis, pleaded guilty to all three counts of rape, three attempted rapes and one act of sexual aggression when his trial opened on Monday .
His twin brother Elvin was also at the court in Aix-en-Provence.
Elvin had been held in custody for 10 months along with Yoan after the pair were arrested by police probing a series of sex attacks in the Marseille area between September 2012 February of the following year.
Both initially denied any wrongdoing
The victims had not able to tell the men apart in photographs police showed them, and as the pair had almost exactly the same DNA, officials decided to err on the side of caution and keep both in detention.
Yoan was eventually identified as the alleged aggressor after some victims recognised Yoan’s speech impediment, which is due to a hearing impairment which his twin did not share. Elvin was released.
The suspect later made a partial confession, admitting to attacking the women but not to sexual assault.
In court on Monday, he made a full confession, also admitting to the sexual nature of his attacks.
"I am sorry for having lied all this time,” Yoan told the court. “I was ashamed of myself, I just couldn’t admit it.”
Sentencing was due later in the week.
It is technically possible to determine differences in identical twins’ genetic code, but the process is very expensive.