As I understand it, multiple castings of the same spell do not stack. For example if you cast a spell with "Fire damage 3 pts for 10 sec on touch/target" and then cast the same spell on the same enemy 5 seconds later, you have done 45 fire damage, not 60. Correct me if this is wrong.

Now, I'm assuming that this also applies to weapon enchantments, at least with effects with multi-second durations. If you have a weapon enchanted with "Fire damage 3 pts for 10 sec on strike" and hit an enemy twice in succession (as you would generally want to do with an unenchanted weapon), the amount of fire damage you've dealt is a lot closer to 30 than 60. Assuming this is indeed the case, then my real question is, what happens when the time between strikes is measured in fractions of a second. Or moreover, does this issue even matter at all when a weapon enchantment has a duration of only 1 second?

For example, daggers are pretty fast, and can easily land more than one hit per second, right? So if you have a dagger enchanted with "Fire damage 10 pts for 1 sec on strike," and you hit an enemy two times exactly half a second apart, then how much fire damage have you done? 10? 15? 20?

Thanks.

A dagger can hit much faster than once per second and if enchanted with 10 fire damage x 1 second, will reset the damage resulting in less than efficient usage.

Long swords and claymores avoid this problem because they hit about once per second or slower.

The shortest damage you can enchant at the altar is over one full second which is not ideal for daggers. Sigil stone damage however is delivered all at once instantly instead of over one second. This aspect of sigils makes them perhaps desirable for dagger enchanting. The drawback with sigils of course is that you are limited to only one enchantment effect on a weapon. Most of the pre-enchanted weapons you find in the game deliver their damage instantly, so if you find a pre-enchanted dagger, that may be worth looking at.

See Acadian's post for the mechanics.

The answer to this question is 15.