Could you please provide all the output?
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Looking at the output this on prefix? I'm wondering about the different paths in the portage output, where one mentions a prefix of /apps/gentoo and the other does not.
In any case /usr/bin/rdoc21 should be installed only by dev-ruby/rdoc. I'm not aware of any bugs related to this for ruby 2.1.10. Perhaps the best course of action would be to re-emerge ruby 2.1.10 and see if this fixes the situation.
I will try to test after overcoming bug #633358.