I had no problem searching for "powerline duplex" to get that info through DuchDuckGo and Google. You should find more info by searching for Homeplug as the full name is the Homeplug Powerline Alliance whereas Powerline is a more generic term. There used to be products that weren't Homeplug compliant - not sure if that is still the case.

It is product dependent because full-duplex is supported by the chipset. From memory, Netgear has full-duplex devices and there are others. Do you have a specific product or are you just asking in general?