I'm just curious... in the stats world would you just assume that duplicate entires in a set of data are really different entities but the set just didn't include spurious attributes like, in this case, gas tank size?
It seems like that would make it hard to verify that the stats on a data set like that were reliable.
Maybe this data set was just put together just to serve to as an example for trying out R? That would be sensible... keep examples as simple as possible for the tasks you are trying to illustrate
BTW the reason for my comment about duplicates is that E. F. Codd, the mathematician who invented relational databases to intrinsically insure the integrity of data, required no duplicates in a table. That's why the dup's in mpg were a surprise to me.
Here is the particular rule
Rule 2: The guaranteed access rule:
Each and every datum (atomic value) in a relational data base is guaranteed to be logically accessible by resorting to a combination of table name, primary key value and column name.
You can find Codd's 12 rules that define the behavior of a relational data base system in bunch of places on the web, here is one: