Looking at http://google.com/url?sa=D&q=https%3A//luci-milo.appspot.com/buildbot/chromeos/hana-paladin/2942

It looks like make install_tools is ran twice:

libmojo-395517-r4: >>> Install libmojo-395517-r4 into /build/hana/tmp/portage/chromeos-base/libmojo-395517-r4/image/ category chromeos-base
libmojo-395517-r4: make -j32 DESTDIR=/build/hana/tmp/portage/chromeos-base/libmojo-395517-r4/image/ install 
libmojo-395517-r4: cd "/build/hana/tmp/portage/chromeos-base/libmojo-395517-r4/work/libmojo-395517/" && make -r -I "/build/hana/tmp/portage/chromeos-base/libmojo-395517-r4/work/libmojo-395517" -f "/build/hana/tmp/portage/chromeos-base/libmojo-395517-r4/work/libmojo-395517/Makefile" SRC="/build/hana/tmp/portage/chromeos-base/libmojo-395517-r4/work/libmojo-395517" OUT="/build/hana/tmp/portage/chromeos-base/libmojo-395517-r4/work/libmojo-395517/" "install"
libmojo-395517-r4: install -d /build/hana/tmp/portage/chromeos-base/libmojo-395517-r4/image//usr/src/libmojo-395517/mojo/
libmojo-395517-r4: make[1]: Entering directory '/build/hana/tmp/portage/chromeos-base/libmojo-395517-r4/work/libmojo-395517'
libmojo-395517-r4: cp -r --preserve=mode /build/hana/tmp/portage/chromeos-base/libmojo-395517-r4/work/libmojo-395517/third_party /build/hana/tmp/portage/chromeos-base/libmojo-395517-r4/image//usr/src/libmojo-395517/
libmojo-395517-r4: cp -r --preserve=mode /build/hana/tmp/portage/chromeos-base/libmojo-395517-r4/work/libmojo-395517/mojo/public/tools/bindings/* \
libmojo-395517-r4: 	/build/hana/tmp/portage/chromeos-base/libmojo-395517-r4/image//usr/src/libmojo-395517/mojo/
....
libmojo-395517-r4: install -d /build/hana/tmp/portage/chromeos-base/libmojo-395517-r4/image//usr/src/libmojo-395517/mojo/
libmojo-395517-r4: install -D -m 755 /build/hana/tmp/portage/chromeos-base/libmojo-395517-r4/work/libmojo-395517//libmojo-395517.pie.a /build/hana/tmp/portage/chromeos-base/libmojo-395517-r4/image//usr/lib/libmojo-395517.pie.a
libmojo-395517-r4: cp -r --preserve=mode /build/hana/tmp/portage/chromeos-base/libmojo-395517-r4/work/libmojo-395517/third_party /build/hana/tmp/portage/chromeos-base/libmojo-395517-r4/image//usr/src/libmojo-395517/
libmojo-395517-r4: cp -r --preserve=mode /build/hana/tmp/portage/chromeos-base/libmojo-395517-r4/work/libmojo-395517/mojo/public/tools/bindings/* \
libmojo-395517-r4: 	/build/hana/tmp/portage/chromeos-base/libmojo-395517-r4/image//usr/src/libmojo-395517/mojo/
libmojo-395517-r4: cp: cannot create directory '/build/hana/tmp/portage/chromeos-base/libmojo-395517-r4/image//usr/src/libmojo-395517/mojo/pylib/mojom_tests': File exists
libmojo-395517-r4: make[1]: *** [/build/hana/tmp/portage/chromeos-base/libmojo-395517-r4/work/libmojo-395517/Makefile:150: install_tool] Error 1
libmojo-395517-r4: make[1]: *** Waiting for unfinished jobs....
libmojo-395517-r4: install -D -m 755 /build/hana/tmp/portage/chromeos-base/libmojo-395517-r4/work/libmojo-395517//libmojo-395517.pic.a /build/hana/tmp/portage/chromeos-base/libmojo-395517-r4/image//usr/lib/libmojo-395517.pic.a


Looking into libmojo Makefile, I would have expected all, clean and install rules to by marked phony, otherwise, I don't see anything wrong.