Preventing duplicate vacuums?

Folks,

Just occurred to me that we have no code to prevent a user from running two 
simultaneos lazy vacuums on the same table.    I can't think of any 
circumstance why running two vacuums would be desirable behavior; how 
difficult would it be to make this an exception?

This becomes a more crucial issue now since the introduction of vacuum_delay 
makes overlapping vacuums more probable.

-- 
-Josh BerkusAglio Database SolutionsSan Francisco