Since he only needed one to receive the appointment, does that mean that he's preventing another student from receiving a nomination?
Not necessarily. In some states and districts, there may not be a sufficient number of qualified applicants to fill a single slate. Thus, multiple MOCs may list the exact same candidates. Take, for example, North Dakota. There are 3 MOCs, which means at least 30 slots in a year (and possibly more if one of the MOCs has 2 noms that year). In some years, there may not be 30 qualified candidates in the state who want to attend a particular SA. Thus, two or even all three MOCs may list some or all of the same candidate(s) on their slates. In that case, you're not taking a slot from someone else b/c there isn't anyone else.
In a competitive region, if one candidate receives 2 noms it MAY mean that another deserving candidate didn't get one. However, it's more complicated than it seems. For example, the Senator thought the candidate was the very best in the state and, in the district, there were only 8 other qualified candidates. In that case, maybe the candidate is "taking" one of the Senator's slots from someone else, but not one of the District's slots b/c they weren't filled.
At the end of the day, the MOCs make their selections. Sometimes they get together to "spread the wealth" in terms of noms and sometimes they don't. There's nothing the candidate can do about it, however it turns out. You earned the nom b/c the MOC considered you the most deserving. If someone else didn't earn that nom, it's really not your issue.
In any event, it's important to "keep" both noms. Here's why. Let's assume Candidate A gets a nom from both the Senator and her Representative. When the SA looks at the Senator's slate, the candidate is ranked (either by the Senator OR by the SA itself), #8 of the 10 candidates on the slate. However, in the District, the candidate is #1 of 10. The higher you rank on a slate, the more likely you are to receive an appointment so, in this case, the odds are looking good based on the Rep's slate but not so good with the Senator. Conversely, in a state where one district is hyper-competitive and the rest of the state not so much, the opposite could happen -- the candidate isn't ranked very high within the district but, when compared to the less competitive candidates on the Senator's slate, is ranked much higher.
Since you don't know your ranking nor do you know anything about the others on your slate(s) -- i.e., their qualifications, their medical status, their interest in the same SA -- you want to keep all of your eggs in the basket.
The above said, being on more than one slate doesn't always improve your chances and I've seen many candidates even in a hyper-competitive area with 2 MOC noms receive turndowns. For example, if you are ranked (by the SA) at the bottom of both slates, you may end up the victim of the "numbers game" and still not receive an appointment. Or, the MOCs may think you're terrific but your teachers and BGO don't and/or the MOC loves you but your record just isn't that strong compared to your competition.
Bottom line: more than one nom is never a bad thing -- sometimes a good thing -- but never determinative (and in some cases, not even "better").