51%ing is probably best explained by the Bitcoin wiki at https://en.bitcoin.it/wiki/Weaknesses#Attacker_has_a_lot_of_computing_power
. There are a few threads that have been devoted to it here on the forum too. At one point BitcoinEXpress threatened to 51% attack namecoin going back thousands of blocks (although I think this involved faking timestamps which as far as I know has been fixed in Litecoin). There's a reasonably straightforward thread about how a 51% attack would work at https://bitcointalk.org/index.php?topic=52388.0
Fairbrix was 51%ed in its first day, which resulted in around 1600 blocks which had been mined becoming orphaned and the 50 Fairbrix block rewards for each of those blocks disappearing from people's wallets, including mine ( https://bitcointalk.org/index.php?topic=46626.60
I don't know of any businesse currently accepting LTC, and can't find anything on the Litecoin wiki about this, but I'd guess/hope that there may be some soon - bear in mind Litecoin is still very young.
According to the mid-price at btc-e, 23 litecoins are currently worth about 19 cents. Personally I'd say hold them at the moment - if things happen like Bitcoin, they could appeciate quite a bit.
Back on topic again...when I replicated the duplicate payout problem I used two different browser windows. A wait feature (say javascipty stuff) on the button is a nice idea but it wouldn't necessarily work, as it wouldn't affect the second browser window.