Hi all. I'm experiencing the same issue. I'm also using nvidia-glx, so, it might be a good lead. I dunno if any of you has been able to circumvent this issue, but I did.
Simply get the libc6-i686 package by hand, and install it using dpkg -i --force-all. After that, I've been able to successfully upgrade libc6 and libc6-dev. I don't know if it's a correct way to handle the issue, but as far as I know, nothing is broken, and all updates are applied :)
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