I bet a second shot wouldn't give the same effect as they'd be expecting it and still on fire from the first anyway.
Actually, according to actual user accounts it's still pretty effective. No real difference than the first time.
But the challenge is this - even a big can has maybe 3 or possibly 4 sustained bursts worth of spray. And that's assuming it's pretty new and still fully pressurized. ANd when people panic a bit, that is often down to 2 sprays worth as they tend to overspray.
There have been several cases of people running out of spray as or just before they reach the safety of their car or home or whatever.
If you read this somewhere i'd be interested in reading also
herrero's and shelton's books are generally accepted as being pretty definitive on the subject, and herrero documents several first hand accounts of the use of sprays and the effects. Especially the 're-attack' problem with black bears. He notes that little issue is more prevelant with blacks than with grizzlies, who tend to not re-engage, but no one knows why.
I've read a lot of case studies where they've used sprays and/or guns. Both have their strenghts, both have their weaknesses. Its' difficult to compare the two accurately because of course there's no way to simulate the results accurately, and no one person will ever be attacked often enough by bears to actually be an 'expert' from first hand experience as to what works best when.
If you're concerned about bears re-attacking, but don' want to carry a firearm, then at least make sure you've got TWO bottles of spray between you. And always get the bigger ones, not the little ones.
If you're going to use sprays, know how to use the spray properly. Don't aim it like a pistol, fire from the hip with the forearm extended forward (you don't want it 'head level' when you fire). Aim by walking the spray onto hte bear - it's a nice wide spray, you won't need to adjust much. Range has to be SHORT - (as in 'you better have big brass ones' short.) Keep your bursts short, no more than 1 second or so, don't panic and use all your spray in one go. And GET THE HELL OUT OF THERE and to safety soonest!
Pros of guns include -
Range. In those rare cases where you have a problem bear at a distance, it's nice to have more than the 20 - 30 feet or so sprays offer. However it's worth noting most successful bear defenses with guns actually happen within pepper spray ranges - there's a lot of evidence to suggest the noise and fire of the gun does as much or more than the bullet to cause the bear to break off the attack.
Fatality - a bear that will attack you would attack the next guy. Nice to just solve the problem. A man-killer bear is a rare thing, removing it from the gene-pool helps keep it more rare. And lets face it - a bear with a gaping sucking wound and a smashed shoulder is much less likely to decide to pick on you again anytime soon.
Cons include -
Must be PROFICIENT, and have practiced extensively with your gun to be able to effectively use it in a self defense situation like that.
Slower to deploy than pepper spray (rifles only, if you're allowed to carry a pistol the pistol is just as fast.)
Can't use it when the bear is already on someone except to scare the bear off with noise, which may or may not work.
Not legal in all parks etc.
Pro's of sprays include -
Fast to deploy. You don't even need to take it out of it's holder to use it effectively, especially if you carry it in a 'baldric' holster.
Cheap. Easy to equip everyone there with one.
Easier to use and train people with (BUT STILL REQUIRES SOME TRAINING OR AT LEAST SOME INSTRUCTION!)
At least as effective as guns at stopping an attack (but doesn't prevent being attacked again).
Can be carried everywhere, except some aircraft.
Can be used on an animal even if it's on top of someone else.
Cons include -
Non lethal - bear may re-attack.
Short range.
You will likely get some in your eyes if you use it. Contrary to some theories, it will NOT be enough to disable or seriously impede you, but it WILL sting and be unplesant.
Impossible to 'test' a canister before you need it. Once fired, even one burst, the cans tend to lose pressure over time so if you test fire one today, you can't trust it next week. That means you have to hope you've got a properly charged bottle with you when the bear attacks.