private static void testDuplicateRoutes(Route... routes) {
        assertThatThrownBy(() -> Routers.routers(ImmutableList.copyOf(routes), null, null,
                                                 Function.identity(), REJECT, false))
                .isInstanceOf(IllegalStateException.class)
                .hasMessageStartingWith("duplicate route:");
    }
Should detect the duplicates even if the mappings are split into more than one router.
core/src/test/java/com/linecorp/armeria/server/RouterTest.java