I would have the first button have a small Javascripty-doo that changes the hidden runmode field from ActuallySubmit to AddAnother, then submit.

dragonchild, I've never used C::A (or C::P), but why must the runmode be in a hidden field? Why not have the submit button be the runmode value? For example:

<form action="blah" method="post">
  <input type="text" name="data" value="Default data." />
  <input type="submit" name="runmode" value="Save" />
  <input type="submit" name="runmode" value="Add more data" />
</form>
[download]

If the "Save" button is clicked, the "Save" runmode will be called. If the "Add more data" button is clicked, that runmode will be called.

The only limitation is that either your button text or runmode names will be ugly.