R. Eliezer b. Yaakov says: "and to serve Him with all your heart": This is an exhortation to the Cohanim that their hearts not be "preoccupied" at the time of the (sacrificial) service.
"with all your heart and with all your soul": Now is it not already written (Ibid. 6:5) "with all your heart and with all your soul"? Here (above) — vis-à-vis the individual; there — (Devarim 11:13) vis-à-vis the congregation. Here, for learning; there, for doing. If you hearken and do what is yours (to do), I, likewise, will do what is Mine, (Ibid. 11:19) "and I shall give the rain of your land, etc."