It's confusing whether one should put things in gemspec development_dependencies or in Gemfile or in both.

Duplication is bad since the lists could get out of sync. And the gemspec's development_dependencies should be a complete list. Therefore, my opinion is that that should be the canonical list and therefore the only list.

Actually, what good is gemspec's development_dependencies? A contributor should clone the repo, run bundle, and get the dev dependencies that way. Therefore development_dependencies is unneeded and you should only list them in Gemfile.

It is simpler to just use Gemfile, since it is a more familiar format. You can copy and paste content into it. For example, if you extract a gem out of an app, you may wan to copy/move some gems from app's Gemfile into new gem's Gemfile. It also generates a Gemfile.lock (which you shouldn't add to git).