Spelling (PR#6570)

Unfortunately, with English it's conceivable that they're both correct.  
But the double 't' sounds/looks *more* correct to me.
Of course, this is coming from a native English speaker who is generally 
unfamiliar with the rules of English.

On the other hand:

test --> testable?
arrest --> arrestable?
contest --> contestable?
detest --> detestable?
...

I expect this case is more like "upset," and I think there are four 
important rules.

1/ Is it being used as a noun, adjective, or verb?

2/ It may be done differently in England than it is in the colonies, but 
you can't count on it.

3/ No matter which way you do it you are certain to upset someone.

4/ If you do it both ways you may appease everyone, but more likely you 
will upset everyone.

HTH,
(I vote for two t's.)
Paul