Yes i tested it and it works
It has to do with how Niotron handles the name collisions for dx/d8. In a previous test dealing with two extensions that included libraries, it was noted that Niotron could build the app but App Inventor couldn't due to the same class being added twice. By examining the output APK, it became clear they were just picking one copy of the class file and including it (how they determine which one is picked is not clear since AFAIK it is closed source... it may just be last version wins). I suspect that is the same issue here, or at least similar. Kawa was unable to resolve which class to actual use so it just throws up its hands for the developer to figure it out. Obviously throwing out one copy of the extension fixes the problem so long as the two versions aren't significantly different enough to cause problems in the user code (e.g., if a new block was added in version 9 from version 7 and version 9 was deleted instead).
Edit: I think it was actually that the two libraries used proguard to obfuscate, so the final extensions both ended up with a class a due to obfuscation and then that caused issues. App Inventor wouldn't compile it but in Niotron class a was included but only from one of the extensions, breaking the other extension since its class a wasn't there.
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Wow, what a sophisticated and unexpected explanation. Thank you.
And so, what must i do to make my project to be built again ? because it's my main problem...
what is strange is that AI2 succeded not long ago to built them with the same version of my extension and now it can't again...
I'm sorry but this is patently false. App Inventor has never been able to compile a project with two extensions that define two components with the same name. Most likely based on the project contents, you tried updating an extension and the package name changed.
Didn't I say something like that?
@jean-marc_Latour But the problem seems to be solved by removing the AppyBuilder extension, right?
OK. I am not efficient enough to understand what to do, but with the image of Anke it is clearer.
Indeed it works like this.
Use WinRAR or 7-Zip to open the project (no need to remane the aia to zip).
sorry. I don't understand why and how, but the aia I gave you (this one on my extension post) was built and it worked on my phone....
I effectyvely modified my extension for an other app but i didn't yet change it on this post...
Anyway removing AppyBuilder version it solve my problem...so thanks a lot to all of you for your help.
I will replace the file in my extension post in order other people can use it...
yes it is what i did. I didn't know that aia file was a zip folder...We learn every day 
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Actually, the solution comes from Evan, but I think he can live with it. 
yes you're right (my apologies to him) but i thought that your post would be more helpfull for people having the same problem