Don't make any sense out of what I am saying. It is what it is.
I am a California general contractor, and I often notice 15 amp-rated switches and duplex receptacles on 20 amp circuits. Is this permitted by NEC or by current California building electric codes, and if so in which cases?
Actually kitchen and laundry rooms are generally required to be on 20A circuits.
Given 15A circuits, 20A receptacles are NOT allowed.
Given 20A circuits, 15A receptacles are allowed provided that there are two or more. A duplex is counted as two. 20A receptacles are also allowed. It is still possible to completely load a 20A circuit through a 3-outlet 16 AWG extension cord and overload the extension cord. The primary purpose of - | shaped outlet is to keep appliances that require over 12A continuous from getting plugged into the wall.
Given other higher amp circuits, lower amp receptacles are not allowed. Example, 20A outlets on a 30A circuit even with multiple outlets isn't allowed.