This happened in a pathological case in Nixpkgs: the "grub" job is evaluated for i686-linux and x86_64-linux, but in the latter case it returns the same derivation as in the former case. So only one build should be added.
JOYONH2KARKK2Z72WQW6DZ4B6WT47Z4VHOHGL7KM3B6RT3CBAMLQC
RXVJFQ5AV3WME4HDVBPSRCALQTXROT4KQPOQVO6KTWTBNZIZZGPAC
3XTHEUMP2ZOMPQWE3S5QWHIHCEJNEXGDPQB3JUVZFPS3RFMY455QC
LBNVQXUBEZ45SOTGVXK5UEZXIAIZTJLWZNUYFI4JZ6J65N3KPDVQC
OOQ2D3KCLFPYNAN253PHWLBQMB6OMO2KYQWQXLTP65SQAYZWQ5LAC
A22P7HCOGNO3XEQWBNEG2VHTGUB2GKQ75Q6JDMPQNQCLFHE7XBFAC
RFE6T5LGBFFNEPHZOPF4UNMFC2L4CGD5TPAMOXDLRPH3TZJ43UBAC
S5PV6IIMKJ7PGWIFLLXERHYF3BCP2UEGFRZEZLD6UUBLVEZXJLUAC
N22GPKYTOLZLBGTGDATQDVZ4R5APZEAOIA7L32X4UXBH4XNI7MWAC