Where else in the game does it allow you to double dip on a consequence or other aspect on a single effect?
How is it double dipping? There are two mechanics applying here. First, you may inflict consequences to contribute their shift value to a spell. Second, you may invoke aspects (if justifyable) to contribute +2 shifts to the spell. Under the standard thaumaturgy rules, I read it as legitimate to inflict, say, a serious consequence on yourself for +6 shifts, then spend a Fate to invoke the resulting aspect for another +2. This assumes that you can justify drawing on the aspect to benefit the spell, and that you have too much available Fate and too few available/appropriate aspects.
I can see the argument about timing, and agree that it's not entirely clearly defined. However, it does state in the Death Curse sidebar that Death Curses count as rituals but with the normal preparation replaced by the circumstances of death (can only tag and inflict consequences, so no sitting out scenes!). But the general thaumaturgy rules allow the various methods of preparation to be "combined in any way you choose" (YS268), which does not appear to be over-ruled in the sidebar.
That said, even though I think the rules allow for you to inflict a consequence then tag it, I think that balance would be better served by allowing the character to do only one or the other. Meh, tables can work this out as they want. However, the two likely resolutions work out to:
1) the shift value of all unused consequences, plus two shifts for each used consequence (assuming the order matters) OR
2) the shift value of all unused consequences, plus two shifts for every consequence slot on the sheet (assuming you can use either/both options in the order you see fit)
Take your pick.
As an interesting aside, either option results in stronger death curses from wizards who happen to have extra consequences (from high ratings in the stress-track skills or certain stunts). Just having the Resilient Self-Image stunt, for example, adds at least +4 (and possibly up to +8, with option 2 above) to the potential strength of a death curse.