With the cache populated by the stdlib, cd-ing to the harec repo and attempting to build (with -R, to avoid further breakage from trying to pull in additional modules) a module with a single empty function, I get:

(.text+0x0): multiple definition of _start'; /home/bor/.cache/hare/home/bor/src/hare/harec/rt/6fbb542519dc7c019449a0195799a08d4edad83f8a08fb433d625f5e13fe4133.o:(.text+0x0): first defined here

and a bunch of other similar ones for syscall[0-6].

I don't understand the build driver very well, but what I expect it to do is figure out the rt module that it's attempting to pull in (the one from harec) is different than the rt module it has in the cache (from the stdlib), and to ignore the stdlib one completely.