Quentin Tarantino’s films are part of the same universe in two different levels, and while some characters are somehow connected (like the Vega brothers, Vic and Vincent), it doesn’t mean that all characters played by the same actor are connected – like Brad Pitt’s characters in Inglourious Basterds and Once Upon a Time in Hollywood. Tarantino’s shared universe began in 1992 with Reservoir Dogs and includes every movie directed by him ever since, as well as Robert Rodriguez’s From Dusk Till Dawn, which Tarantino wrote and produced.
Throughout his career, Tarantino has made films from different genres, such as the exploitation horror film Death Proof and the western thriller The Hateful Eight. His most recent work is Once Upon a Time in Hollywood, a film that, just like Inglourious Basterds, took real-life events and retold them in a more satisfying (and fair) way. Aside from that, both have another element in common: Brad Pitt, which has made some viewers believe that he’s playing the same character.
Pitt played Lieutenant Aldo Raine in Inglourious Basterds, which is set in 1944. Raine was the leader of the “Basterds”, a group whose sole mission was to kill Nazis. Raine had a large scar around his neck which was supposedly the result of an attempted lynching while fighting the KKK. In Once Upon a Time in Hollywood, Pitt played Cliff Booth, a stuntman who served in war and was later struggling (along with his friend, Rick Dalton) to stay active during the last years of Hollywood’s Golden Age. The film is set in 1969, so in terms of timeline, it's possible that Raine later took on a different identity and became a stuntman. A detail that the believers of this theory use to support it is that Raine, when going undercover to the premiere of Goebbels’ latest film, posed as an Italian stuntman named “Enzo Gorlami”. Raine probably liked the idea a bit too much and became a stuntman years later, but there’s a very important detail fans seem to be forgetting.
As mentioned above, Aldo Raine had a very visible scar on his neck – Cliff Booth didn’t have any. Because there’s no information on when Raine and Booth were born, it’s not possible to place them on the Tarantino-verse timeline, but given that Brad Pitt doesn’t seem to age, the “Raine and Booth are the same person” theory could work – except for the scar, which is really the biggest detail that makes the whole idea not possible. Still, it’s fun to imagine Aldo Raine going after Nazis and then fighting Charles Manson’s followers when they broke into Dalton’s home.
Unless Tarantino himself comments on it and confirms or denies this theory/idea, Aldo Raine and Cliff Booth will remain as separate characters. Both Inglourious Basterds and Once Upon a Time in Hollywood are part of the “real world” level of Tarantino’s film universe, so there are probably other links between them, though not exactly involving Brad Pitt’s characters.