Per 210-21(b), would you consider a duplex outlet a single receptacle?
Can a 15 Amp duplex be installed on a 20 Amp circuit without any other outlets?
(I've got somebody that won't believe me)

The FPN refers us to article 100. (Definition of receptacle)

“A single receptacle is a single contact device with no other contact device on the same yoke. A multiple receptacle is two or more contact devices on the same yoke.”

A duplex is a multiple outlet device by definition.

Yes on 15A duplex receptacle can be protected by a 20A breaker.

Nick,

I am not sure but if it is a dedicated 20 amp line, you must use a 20amp recp.I could be wrong.

Only if it's a 'single' receptacle. A duplex is not a single receptacle by definition.

Ditto Nick. A duplex is 2 receptacles and, therefore a 15 amp is acceptable, even on a dedicated line.

Thanks, fellers. Although I originally showed the "stuporintendent" the exact same chain that Nick mentioned, he wouldn't agree. I asked you, printed this thread, and brought it in to him. He's now shut up but he's mad :

If you take a duplex, break off the hot and neutral tabs, and feed two circuits into it, would it be considered 2 single outlet circuits?

It still would not meet the definition of a single receptacle because it is still two devices on one yoke.