My permissions post may affect the answer to #1, actually. Although I'm pretty sure I've guessed right.
1. Greed would not directly spot a duplicate, but Ling would probably know right away exactly which one was real. Whether he would actually communicate this information to Greed in a timely fashion depends on the circumstances. 2. Although technically he's not human, Greed's "body structure and compositional elements" are totally identical, so this should work just fine. The Ultimate Shield would probably block it, though, if he's using it at the time. 3. Absolutely! Do eeeeeet! 4. Greed is sharing a body with someone else. He's also filled to the brim with energy, in the form of a Philosopher's Stone, although he lacks the ability to consciously use it in all the ways it could be used. None of this is hidden in any way. 5. Be as mean as you like to him! I can't think of any particular sore spots for me.
For Greedling
1. Greed would not directly spot a duplicate, but Ling would probably know right away exactly which one was real. Whether he would actually communicate this information to Greed in a timely fashion depends on the circumstances.
2. Although technically he's not human, Greed's "body structure and compositional elements" are totally identical, so this should work just fine. The Ultimate Shield would probably block it, though, if he's using it at the time.
3. Absolutely! Do eeeeeet!
4. Greed is sharing a body with someone else. He's also filled to the brim with energy, in the form of a Philosopher's Stone, although he lacks the ability to consciously use it in all the ways it could be used. None of this is hidden in any way.
5. Be as mean as you like to him! I can't think of any particular sore spots for me.