...and in 2.6x:

http://wiki.blender.org/index.php/Doc:2.6/Manual/Modeling/Objects/Duplication

But the question begs asking - are we referring to deliberate duplication of faces or:

http://www.katsbits.com/tutorials/blender/double-sided-faces-different-materials.php

Do you have a link to that tutorial? Might go have a look real-quick... - ETA... never mind, found it.

BTW, agree with you: videos are great but sometimes do move much too quickly, and you can't index them (well, you can, sort-of, but it's awkward) and it's virtually impossible to search a video.

Monterey/Mint21.x/Win10 - Blender3.x - PP11.3(cm) - Musescore3.6.2

Wir sind gewohnt, daß die Menschen verhöhnen was sie nicht verstehen
[it is clear that humans have contempt for that which they do not understand] 

Metaphor of Chooks