A rare example tends to happen in Ayreon concerts, where multiple vocalists were used to represent different characters before, they have a drastically reduced cast. This has led to some amusing moments where you can hear a Me from The Human Equation asking AND answering his own questions.
An acoustic version of "Castle Hall" features a single singer playing both the Barbarian being horrified by the ghost of the women he raped and the Knight calling on Merlin, Excalibur and the Knight of the Round table to help him.
E=MC2 which originally featured 5 fish people aliens arguing over whether to save Earth from a course of destruction they set it on; however, the acoustic version features Irene Jansen have a near breakdown arguing with herself over the same query.
In 2006, Aled Jones made a recording alongside his boy treble self with a new rendition of "Walking in the Air."
The lyrics to Thank You Pain by The Agonist consist of a back-and-forth between a judge and a defendant. Both are voiced by the same singer, who uses growling vocals for the judge's lines and clean vocals for the defendant.
Ray Stevens is fond of voicing multiple characters in the same song:
In "The Dooright Family," he voices all five members of a fictional gospel family band.
In "Dudley Doright of the Highway Patrol," he has a conversation between himself and the title character, also voiced by himself.
In "Theme from The Monkees," he voices an entire Austrian singing troupe and a narrator.
In "Gitarzan," he voices the title character, Jane and the monkey, all of whom sing together at the end.
In "Moonlight Special," he voices the host of a fictional rock & roll show and all the "acts" on it. This includes a hilarious Lampshade Hanging in which a lead singer questions why the backing vocalists keep repeating her.
In "The Streak", he's both the reporter, and the guy being interviewed.
Brad Paisley's "Born on Christmas Day" includes elements from a recording that he made at age 13. On the final chorus, he and his 13-year-old self sing together.
Jeff Foxworthy has a conversation with himself in "The Redneck 12 Days of Christmas:"
Jeff's other voice: These ain't normal Christmas presents. Jeff himself: No, they're redneck gifts. Other voice: Redneck gifts?
In Pink Floyd's The Wall, the last full-length song, "The Trial", has lots of characters, all voiced by Roger Waters.
Bryn Terfel's "Bad Boys" album includes a scene from Don Giovanni that features the titular character, his manservant and his ghostly nemesis. All three parts are within Terfel's vocal range, and he duly performs all of them, singing to himself in triplicate.
Postive K's "I Got A Man" consists of a rapped conversation where a man tries to pick up a woman and she very bluntly rejects him. Both parts are played by Positive K himself, with added pitch-shifting to make the woman's voice more convincing. For the music video, they had an actress lip-syncing the woman's lines.
"Keep Punching Joe" by Daniel Johnston begins with "someone" introducing Daniel Johnston, followed by Daniel thanking the person and then he starts singing. Both voices clearly belong to the same person.
In the title track of Rush's 2112, Geddy Lee voices both the unnamed protagonist and the priest in an argument about music.
In the Dream Theater song "The Case That Stumped Them All", vocalist James LaBrie portrays a baffled doctor and a female nurse who trade vocal lines during the verses. The result are...slightly disturbing. Their concept album Scenes from a Memory could also be an example as he plays most of the characters when singing, but he doesn't try to do special voices for them.
Two recordings of the opera Boris Godunov have Boris, Varlaam and Pimen all played by Boris Christoff. This would not be possible for a singer to do on stage because Boris's death scene begins with him conversing with Pimen.
David Bowie is both Ground Control and Major Tom in "Space Oddity."
Eminem often talks to himself in songs, especially eariler ones such as "My Name Is."