lmodern aka lm is now installed by both dev-texlive/texlive-luatex and dev-texlive/texlive-fontsrecommended Reproducible: Always
bug #730006 is a duplicate that contains actual build logs.
Created attachment 646992 [details] Failed build log
I can confirm this bug, attached build log. dev-texlive/texlive-fontsrecommended conflicts with dev-texlive/texlive-luatex. In my case dev-texlive/texlive-luatex was pulled by dev-python/matplotlib with "latex" use flag and dev-texlive/texlive-fontsrecommended was pulled in by app-text/texlive
*** This bug has been marked as a duplicate of bug 730006 ***