vendor invoices problems

[Victor Sterpu <..hidden..>]

The back button from the browser creates a lot of problems.
When a new invoice is created operator can press "Post", then the "Back" 
button and then "Post" again.
This heappens to my users very often.
To prevent this I created a index:
CREATE UNIQUE INDEX ap_transdate_vendorid_invnumber ON ap(transdate, 
vendor_id, invnumber);

Can we include this index in the next version of the ledgersmb?