/usr/share/eigen3/cmake/Eigen3Config.cmake contains the following lines which
are clearly not correct:

set (EIGEN3_INCLUDE_DIR  "${PACKAGE_PREFIX_DIR}/${PACKAGE_PREFIX_DIR}/include")
set (EIGEN3_INCLUDE_DIRS "${PACKAGE_PREFIX_DIR}/${PACKAGE_PREFIX_DIR}/include")

and this results in cmake determining EIGEN3_INCLUDE_DIR to be
/usr//usr/include/eigen3.

I think this all comes from:

https://build.opensuse.org/package/view_file/openSUSE:Leap:15.1/eigen3/eigen3.spec?expand=1

where there appears to be a misunderstanding as to what -DINCLUDE_INSTALL_DIR
is specifying.