Fix maybe?

    if (spellid1 == spellid2) {
        if (sp1.dot_stacking_exempt == 1 && caster1 != caster2) { // same caster can refresh
            mlog(SPELLS__STACKING, "Blocking spell due to dot stacking exemption.");
            return -1;
        } else if (spellid1 == 2751) {
            mlog(SPELLS__STACKING, "Blocking spell because manaburn does not stack with itself.");
            return -1;
        }
        bool overwrite_complete_match = true;
        for (i = 0; i < EFFECT_COUNT; i++) {
            if (!IsBlankSpellEffect(spellid1, i)) {
                overwrite_complete_match = false;
                break;
            }
        }
        if(overwrite_complete_match)
            return 1; //Spell is completely blank and matches in ID so we overwrite!
    }