Because casting it again will just stack durations, and the duration is as long as you want.
Yes,but I don't care about duration.....
So you say that when my second body casts Astral projection it just dissappears?Where are you getting this ruling?
I'll give you two choices.
1. you assume counts under stacking effects and that therefor it doesn't work. (combining magical effects, PH 171)
2. you pull off a stupid infinite loop that gives the DM increasing places to kill you, and increasing reason to kill you.
Because abusing the copy effect would likely count as violating a natural law, inevitables will begin to target you, and your sliver cords only have hardness 10 and 20 hp.
some quick numbers
[spoiler]Assuming 1 of the least effective ones, it would take 533.3 attacks assuming it needs a 20 to hit. that's 26 minutes and 40 seconds.
the most effective one would deal 2d6+12+3d6-10 damage (7 to 32) damage in the first swing, and the second swing would almost total assure death. even if it needs 20 to hit, that's only 2 minutes.
the second most efficient will average death in 8 hits. at two attacks per round and needing 20 to hit it's still only 8 minutes.[/spoiler]
Do you really want a CR 9 creature to kill you that easy?