I agree it would be handy sometimes, but it would probably slow things down. This site is far busier than most.
The ability to delete duplicate threads might save some load on the servers, but I suspect that the cons would outweigh the pros, otherwise it would have been implemented already.
No use asking really. The forum's designers will have heard and considered all such wishes before, and will have designed it as they think best. Making changes to such a huge site is a mammoth task, and it has be updated twice recently.
It is good to get into the habit of spell-checking before posting. Opera 7.50 now has spell-check with Aspell. If you need a spell-checker for IE try Ispell or Tiny Spell.