What if iam native speaker of spanish and i level 9?
It dosen't make any sense!
I will assume that what you mean is that you took a placement test and it only took you to level 9. The tests only take you to level 10 at most. They are supposed to exempt you from certain lessons while giving you the XP points you would have been awarded for them, but they do not substitute for the strengthening lessons that you didn't require.
That is the reason for it, but I must say I also think it should take you even higher in level. We are native speakers after all. Those practice lessons were part of our lives since the beginning.
Duolingo does not need to know you're a native speaker, since its courses can only take you to level B1 at most, that should be no problem for a native speaker.
You only need to get all answers correct for it to know you're
well-versed, which would be equivalent to you having completed and successfully assimilated the whole course after multiple practices.
If you make too many errors in your starting language, Duo doesn't know whether your problem stems from your base language or your target language. Plus, the levels have nothing to do with your competence, just the number of points you gain through practice. If it takes you to level nine to start with, it means you have passed (once) through that number of lessons to gain the 10 points per lesson. What is more important in judging competency is the number of skills you pass either partially or completely.
Well, if you are a native Spanish speaker and you are taking the Spanish course for English speakers then what you are doing is called "the reverse tree" here and it is considered to be even harder to go through. So there's still room for learning here.
Of course if you are already fluent in English too, then it woudn't be difficult either way.