Midrash su Esodo 30:38

7) Likewise, R. Yehudah says (Shemoth 30:18): "And you shall make a laver of copper, and its pedestal, of copper, for washing": I might think that the pedestal could serve for ablution just as the laver does; it is, therefore, written "copper," "copper" (twice) — it is likened (to the laver) only in respect of its being copper, and not in respect to washing. Likewise, R. Yehudah says (Shemot 28:15): "And you shall make a breastplate (choshen) of judgment, artistic work; like the work of the ephod shall you make it." I might think that just as this (the choshen) was doubled, so, that (the ephod) was to be doubled; it is, therefore, written: "of gold": It (the ephod) is similar to it (the choshen) only in respect to gold, but not in respect to doubling.