Following blakem's rules, 
#        1         2         3       
#234567890123456789012345678901234567
$"='';grep{/^(([@_])(?!.*\2))+$/}<D>;

[download]
37 characters! I'm pleased with my first try at golfing. ;)

andy.

PS stuffy, good choice of puzzle! There went my lunchtime.

update: 36 

#234567890123456789012345678901234567
$"='';grep/^(([@_])(?!.*\2))+$/,<D>;

[download]
update 2: Thanks dragonchild, 35 w/out the ;
Hmm... you're right about the duplicate letters. I can't remember whether these appear on the t.v. show - I rather suspect they do.

update, later:
This cope with duplicates, but is a lot longer:

#        1         2         3         4         5         6         7
+         8
#234567890123456789012345678901234567890123456789012345678901234567890
+12345678901
$"='';$n{$_}++for@_;grep/^((??{"([@_])"})(??{"(?!([^$1 ]*$1){$n{$1},})
+"}))+$/,<D>

[download]
(you need to use re 'eval')
I'd rather do:
$"='';$n{$_}++for@_;grep/^(([@_])(??{"(?!([^$1 ]*$1){$n{$1},})"}))+$/,
+<D>

[download]
(the difference being that @_ is interpolated normally here), but for some reason this isn't allowed - I get back Eval-group in insecure regular expression in regex - no idea why, since I'm using re 'eval', and @_ surely can't be tainted?

Going to dinner. andy.

In reply to andye Re: "Countdown" (golf) by andye
in thread "Countdown" (golf) by stuffy

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