In the early seventies, DJ Kool Herc was one of the originators of hip-hop through his innovative use of two copies of the same record on dual turntables. He and his fans were tired of the then-popular, lush, over-produced, over-packed sounds of disco and funk music, but they did love the "break" sections, which provided a contrast in many songs with a more minimal, drum and bass focused sound.
By using two copies, he could play the breakbeat on one turntable while resetting the other turntable back to the start of the breakbeat, and thus keep it going as long as he wanted. Also using duplicate records, Grandmaster Flash perfected the same technique to the point where he could keep the beat going without any interruption in the rhythm. In the studio, a similar effect is achieved using sampling (originally done with tape loops), but live DJs continue to extend sections, or even entire songs, as long as they want, with the use of dual copies of a record. (If the record has an instrumental on one side, they can also fade the vocals in and out, and mix in other sounds, with a variation on this technique).
It's worth noting that this practice was the origin both of "break"-dancing (dancing done to the breakbeat section) and of rap music (originated by street poets improvising impromptu verses over the space opened up in the music by extending the break section). The recent Netflix series The GetDown provides an entertaining dramatization of this period in music, and the origins of the hip-hop sound and culture.