on iTunes, do you list both artists as the main artist? Do you make your favourite the main artist and add the other as a feature? Do you make your favourite the main artist and add the other as a duet? Do you make the album artist the official artist and add the other as a feature/duet? Do you completely remove half of the duo from your tags?

examples:
"If I Never Knew You" by Jon Secada & Shanice
"If I Never Knew You (feat. Jon Secade)" by Shanice
"If I Never Knew You (Duet with Jon Secade)" by Shanice

I see this done in different ways on last.fm and in the iTunes store, and was wondering which style you guys preferred.

I put both artists as the main (Artist & Artist) unless it's from a duet album, then I put (Duet with Artist).

How they're labeled when I buy them on iTunes. Why would I change it?

Some people change tags to make their iTunes and/or last.fm more organized to their liking.

Usually just leave it the same as the official iTunes unless my fave is involved then its Song Title (feat. random) - My Fave

One verse or the chorus = feature - artist featuring artist

(Mariah Carey featuring Young Jeezy)
(JaRule featuring Christina Milian)

ACTUAL duet = duet - Artist AND Artist

(Brandy and Monica)
(Teddy Pendergrass and Stephanie Mills)

If it's a collab, Song (feat. Artist II) - Artist I

If it's a real duet, Song - Artist I & Artist II

Basically

I go with the feat. option. The main artist is given to whoever I like the best

It basically comes down on how it was listed on iTunes.

For example:

Katy Perry - Dark Horse (feat. Juicy J)
Tony Bennett & Amy Winehouse - Body and Soul

However they are labelled on the artwork/booklet of the actual CD.

"artist I prefer" - "song name"

Although when it's just a song feat a rapper then "artist" - "song title (feat. artist)"

If one was featured then Song Title - Artist & Artist

If they shared lead billing them Song Title - Song Title - Artist vs Artist

Now I use Queen Spotify though

I'm not talking about rap features, I'm more speaking on terms of Bang Bang and When You Believe.

A Great Big World & Christina Aguilera - Say Something
Kristen Bell & Santino Fontana - Love Is An Open Door
DJ Snake & Lil Jon - Turn Down For What

(with _____)

Depends on which album it comes from. I usually use (w/ _______). Makes the most sense to me.