Also, these people offer to "teach" me French, when their profiles are poorly written in French. I always reply to them that I'm only looking for fluent French speakers, then they accuse me of being a racist because of the media. And they end te conversation with an insult toward France. I have already reported about six people for this since I started using this site. I'd like Italki to do something about what you are mentioning as well. Thanks for bringing to the fore this issue.

I correct things that have already been corrected, because in my own language learning, I am happy when multiple native speakers correct my work, even when they correct it to the same (or almost the same, not direct copy paste, dead give away for that is having same correction colour), because I know then that that repeated form is the most current, the most logical, transcends dialectal differences and is actually correct. I also sometimes don't read all of the corrections a post has, because I don't want them to influence how I would phrase something. 

Having said that, what is very frustrating is when non-native speakers correct people's work and add NEW mistakes. I see this problem a lot in English and French entries, because those are my native languages and so I spot the mistakes more easily, though I am sure that this is a problem in all major languages. People often over-estimate their skills in a language...  

This is a very common problem especially with the entries written in French. I have had to delete most of my entries in this language because some users who are not clearly native French speakers "correct" them and only add new mistakes (awful spelling errors, they don't know how to conjugate the verbs in past, they get wrong the gender of the nouns). So, as they are the only "corrections" which are not useful at all I have to delete my entries. 

That being said, I do think non-native speakers can teach their second languages. For example, on your teacher profile, I can see that you teach English, even though what you have written here contains a errors. However, for a beginner, how may have never said anything beyong "Hello", your level is perfectly fine. I think that in order to teach or correct anything beyond A2, a non-native speaker should be either "dominant" in that language (for example, someone who was born in Romania but moved to England at the age of two, would probably be more proficient in English than in his or her native language) or posses a university degree in the TEACHING the target, non-native language.

I usually just down vote the entries with mistakes and provide the correct form, even if it is already given, so that the learner can see multiple corrections with the proper form and will hopefully deduce that the other correction contains mistakes, and I don't worry to much about is after that... Limiting who can correct what might make the number of people who never recieve corrections go up further, and that would be just as much of a shame.

Well, I'll have to disagree not because I am saying you are wrong because I don't know how it works in other languages but in my own language Portuguese, most of fluent speakers like Americans, Germans and etc have a much more better understanding of concepts, grammar and rules than the so-called " native speakers ". 

Brazilians suck so bad at their own language and some grammar mistakes we make here are so common that we think they are right! So, I wouldn't judge someone just because they're not native speaker because sometimes being native speakers mean nothing!

As to spanish, I've seen so many native speakers with horrible spelling mistakes where they could not make a difference between " B and V ", but of course that does not apply to everybody!