Can a unit have two instances of the same keyword?

Example: Blizzard Force AT-ST has an Aft Armor Plating attached to it. Upon declaring it as an attacker/defender, do both trigger and therefore allowing me to shield 2 different cards?

It depends on the keyword.

Value dependent keywords (eg Edge) do stack, so a card that gets Edge (1) from 2 different sources would have Edge (2).

Keywords that aren't value dependent (Shielding, Elite, etc) do not stack. Having shielding twice does nothing extra.

Exactly, dbmeboy.

Enumerated things stack, everything else is more like a yes/no quality. If you give Shielding ability to a unit with Shielding...well, it still just has shielding. Same as having two objectives that say your opponent must play the first card in the Edge stack face-up. Opponent still plays the first card face-up, not the first two.

Almost anything with a number, or enumerated, can stack.

Exactly, dbmeboy.

Enumerated things stack, everything else is more like a yes/no quality. If you give Shielding ability to a unit with Shielding...well, it still just has shielding. Same as having two objectives that say your opponent must play the first card in the Edge stack face-up. Opponent still plays the first card face-up, not the first two.

Almost anything with a number, or enumerated, can stack.

While the ruling on double instances of shielding is correct this post can cause some confusion, because most of the objectives actually do stack. matter fact almost all of them do, you just happened to name one that does not. i'm not going to go through a huge list of them because, well, many of them stack, but here is a few that stack: fall of the jedi, counsel of the sith, emperors web, in you must go, forgotten heroes, last minute rescue, decoy at dantooine, message from beyond etc etc and thats just a small sample of them.

The posters above are correct. I just wanted to add that this answer was placed in the most recent FAQ (which is up today) in section 2.11.