Let's Duet

Alice: There’s lots of ways to show your love, But one that can’t go wrong…
Bob: Is looking in your lover’s eyes, And singing a love song!
Both: It’s a duet, A musical composition for two people,
A duet,
There aren’t many words that rhyme with people!
To throw some romance in the mix,
The best way is to add lyrics,
To prove love, lovers don’t use tricks--
They do it in duet.

Self demonstration out of the way, a duet is any piece of music made for two singers or instruments. They show up mainly in music-oriented media, like songs, musicals, and opera.

Due to the intimacy that comes with sharing a song, by far the most common form of duet is the Silly Love Songs type, where the two participants are meant to be lovers. Some composers play on this intimacy to give the duet a humourous or unsettling feel—for example, having two Heterosexual Life Partners sing about their bromance, or giving the hero and the villain a duet to show they’re Not So Different.

Of course, that's only the thematic reason for duets; there are many other reasons for having them, including boosting a song by having two star performers lend their voices to it. Compositionally, duets are nice because the performers can harmonize, adding an extra layer to the song that would otherwise have to be achieved with backup singers.

The Award Bait Song is often a duet. Compare Serenade Your Lover.

Sub-tropes:

Examples of Let's Duet include:

Please only include parodies, lampshades, and particularly interesting examples, since there are so very many straight examples.

Film

The movie Duets is about people singing duets.
Walk Hard has a song about duets actually called "Let's Duet", with more than a few Double Entendres.

Let's duet, in ways that make us feel good.
Let's duet, and make that sacred sound
Put two and two together, perfect harmony we found.
We know it's only natural, let's duet!

In Enchanted, Giselle and Prince Edward sing "True Love's Kiss" together when they meet and fall in love in their fairy-tale world:

Edward: You're the fairest maid I've ever met.
You were made
Giselle: ... To finish your duet!

- It's a sign of her Character Development when, after spending time with Robert in Real Life, she doesn't come in when Edward sings the line.

Live-Action TV

The musical episode of Scrubs has J.D. and Turk sing "Guy Love," a duet about their bond as Heterosexual Life Partners.
Episode 2 of Mongrels has a wonderful duet about the Jail Bait Wait here.
Glee, of course. They even had an entire episode featuring duets that played with the concept. Most of the duets were with romantic couples, but a few were fairly unique—like Kurt's duet, which he sang by himself, and the Mercedes/Satanna duet (they're usually enemies).
Jeeves and Wooster got some comedy out of Bertie getting Jeeves to sing with him while he played a duet on the piano.

Music

The Walk Hard example above is probably inspired by Cole Porter's equally Double Entendre-filled duet, "Let's Do It (Let's Fall in Love)"
"Dueling Banjoes", of all things, is itself a duet.

Tabletop Games

Magic: The Gathering had "Love Song of Night and Day", a duet poem, read by two lovers. The full piece is here; snippets of the poem can be found in the flavortext of several cards across the Mirage and Visions expansions.

Theatre

Lampshaded in "The Song That Goes Like This" from Spamalot.
Mixing this with "The Villain Sucks" Song, "Pore Jud is Daid" in Oklahoma!! is a duet sung by the hero and the villain, containing a bunch of thinly-veiled jabs the villain, who doesn't catch the insults.
How to Succeed In Business Without Really Trying has "It's Been A Long Day", much of which is Smitty (Frump in the reprise) narrating the complementary thoughts going on inside Finch and Rosemary's heads (Frump is the go-between for Hedy LaRue and Biggsley). Part of the song almost has the feel of this trope, but it's played with and twisted around for laughs.

Web Comics

Lamp shaded and parodied in one strip of Questionable Content. When Marten specifically requests that their band not devolve into sappy love songs, his two bandmates (who are dating) break into spontaneous song about how they met at a mosh pit when she knocked out his tooth.
Kevin and Kell sing a song together in song written for Bill (the strip's creator) by Tom Smith. The song retells the story of how they met each other in a chat room and then agreed to see each other in person. More information is available here, and lyrics can be found here.

Web Original

Lampshaded in Commentary! The Musical at the end of "Ten Dollar Solo", which ends up... not being one.
- Don't forget My Eyes from the regular Dr. Horrible.

Joss pays his debt / with this cheesy duet...

Western Animation

The SpongeBob SquarePants song "Without You".
Aladdin has duet in "A Whole New World."
Tangled follows up with "I See The Light." In context, it doesn't melt into love song until Flynn starts singing though one wouldn't assume it listening to it straight.
In the My Little Pony: Friendship Is Magic episode "May the Best Pet Win", Fluttershy and Rainbow Dash start off the songs of Season 2 with an epic duet about picking out the best pet.
- In "A Canterlot Wedding", Cadence and Chrysalis wrap up the season with a legitimately epic Villain Song duet about the latter's plan to imitate the former and use Shining Armor for her own nefarious purposes.
"Dancing on Air", the first in-universe song in the original My Little Pony pilot, is a duet between Megan and Firefly. Up until the end Megan isn't buying Firefly's assurances that she will be of help to the talking ponies.
In the Waynehead episode "No' Mo Money", Damey and Roz starts off the rap song with a rap duet.