Is a duplex outlet's max amperage rating for each socket or the whole unit?
Each socket can supply up to 20 amps.

However, the total draw of everything on the circuit can also not exceed 20 amps.

Therefore, if you only have one device on the circuit, it can draw up to 20A from any one outlet.
However, if you have a 15A device on the circuit, the remaining devices can only draw up to 5A.

Basically, the sum of the current draws for every device on the circuit has to be less then 20A.