Question about spell stacking

This isn't specifically a modding question and I'm sure it's been asked before or is even in the manual, but can a spell be cast on a target that has already been the target of the same spell with the original spell still being active?

If yes, what is the effect of this:

1. Effects stack for their appropriate duration(s).
2. Effect 'starts over' at the time of the casting.
3. Effect continues when current iteration is done, for the new spell's entire duration.
4. Other.

If no, then "OK."