Or HaChaim on Numbers 18:8/אור החיים על במדבר י״ח:ח׳

1 ואני הנני נתתי לד את משמרת תרומותי, "And I, behold I have placed you in charge of My heave-offerings." The reason for the word ואני, which appears superfluous, is that G'd adds another duty to the duty of the priests to guard the sanctity of the Sanctuary on pain of death; G'd states that He had already given the priests an advance payment so to speak, namely the task to preserve the sanctity of the heave-offerings (which are theirs) and the sanctity of everything holy the Israelites designate as such.