Should have thought of "single" and "singleplex" meaning different things.
Let's see if I have this right
If the circuit is dedicated (has only one outlet), the amps must match, period.
On a 20 amp circuit with multiple receptacles, I can mix and match 15 and 20 amp outlets, either singleplex or duplex, to my heart's content.
On a 15 amp circuit with multiple receptacles, I can use any combination of singleplex and duplex receptacles as long as they're designed for a 15 amp circuit.
How am I doing?