dnd 3.5e: What happens when you cast a spell with the spell activated?

As it is written, I think it is not clear (bad wording as you say), what is supposed to happen and what will really depend on the DM to decide. But I will try:

We're going to break the description of the spell a bit. According to him Spell description:

Spells and spell-like effects directed at you are rejected by the original caster.

This DM would rule that Spell I would return the spell, as the description says. Therefore, if you address yourself, you are still the target, but you also lose x levels of Spell doing that.

Now, there is the question of the resonance field:

If you and an attacker cast spell are protected by the spinning effects of spells in operation, a resonance field is created.

The spell does not do it, and it can not differentiate between the throwers (you're not focused on it or anything) and so, as it is written and because the spell does not know aggressor since no attacker, there may be a case for the resonance field effect. Definitely, and it could be something very funny, random.

However, there is also a good case that since there are not really two Spell twists reacting against each other so that this effect does not happen. For this DM, the resonance fields require this set of events:
– a spell cast by the enemy
– returned to the "enemy" by you Spell
– Then he came back to you for his Spell
– creates a resonance field effect (imagine a spell caught between two bubbles exploding in a random effect)

In spite of RAW, this DM would say that there would be no resonance field effect (unless the previous scenario is fulfilled). All this is to say that once you throw SpellDo not expect to sign up with spells or lose Spell levels

Now, if you throw Spell in yourself while you already have Spell above… 😉