J.B.
Our two youngest are just under two years apart (5 & 7) and now that we're past them being in very different stages for a lot of our shopping, we end up just getting things that we know both or either would like and then we sort things out to make them "even" (or equitable). At the end of the day, it all goes into the same playroom, kwim?
When they were younger, we wouldn't buy things that we already had (and I have a son who is 6+ years older than these two so we already had a lot of stuff). So that meant my youngest often got personalized things and gifts like books, puzzles, cars, balls and crafts, which you can never have too many of. If there are any things that came as a set when you bought it for your older son (I'm thinking things like the Little People sets, the Shake and Go Speedway) you can get him some extra people/accessories/cars that will go along with the big set but be his own.
I honestly wouldn't think twice about it if I were you. You may feel like your cheating him, but he won't know or care. He'll be so excited about everything that going overboard buying things that you already have or going out of your way to find things that are unique will be a waste of energy and money. Whenever I feel like we're shortchanging (or spoiling) the youngest one, I remind myself that it all evens out anyway when he gets to be the last one at home and the other are out supporting themselves and/or working hard in college.