seeds.rb isn't intended to be run more than once on a particular database instance. Running it more than once results in duplicate entries, unless you're explicitly checking for dupes (not always possible).
It looks like seedbank is fairly close to what I want though, as it lets you maintain multiple sets of seeds: https://github.com/james2m/seedbank
It looks like seedbank is fairly close to what I want though, as it lets you maintain multiple sets of seeds: https://github.com/james2m/seedbank