This sort of thing should be freeware. It's a great program but so limited in use, scale, and functionality I don't get how it can be shareware. Even at $8.
From my perspective freeware is the kind of app you use once and forget about, or at best occassional use. Shareware, the kind you buy, is an application that does something you're going to need regularly that performs an important maybe even unique function. iTunes already has the basic feature out, iDupe certainly expands upon it and improves it. But how often you use this app significantly undercuts its commercial value.
Unless you operate your computer under dubious patterns checking your iTunes library regularly for duplicates isn't something you're going to do. Maybe once or twice a year, but even that's pushing it.