According to the National Jewish Health website the Duoneb is a combination of Albuterol and Ipratropium. It stated that the latter can be combined with the former if used within an hour.  Since the Duoneb has both in it, the thing that you have done is probably inhaled a higher level of Albuterol than the theraputic level.  There are side effects from taking too much Albuterol, which can have serious side-effects. Overdosage of Albuterol can nervousness, headache, tremors, dry mouth, chest pain or heavy feeling, rapid or uneven heart rate, spreading pain to the arm and shoulder, nausea, sweating, dizziness, seizures, light headedness/fainting (Drugs.com).  If you haven't experienced this by now, then you are probably out of the woods, but I certainly wouldn't recommend that you repead the mistake.