[gentoo-alt] QA notice : double prefix when updating portage

When I update portage from 2.1.20.4765 to 2.1.20.4871 I've a lot of output messages like this : "QA Notice : //Library/gentoo/path double prefix". Emerge stop with this message :
"!!! ERROR: sys-apps/portage- 2.1.20.4871 failed.
Call stack:
  misc-functions.sh, line 447:   Called install_qa_check
  misc-functions.sh, line 190:   Called die

!!! Aborting due to QA concerns: 163 double prefix files installed
!!! If you need support, post the topmost build error, and the call stack if relevant.

!!! install_qa_check failed; exiting."

I searched into the mailing list to know if this problem hapened and I see only one similar with ruby/gems.

If someone can help me ... :)

Thanks.