Warn about duplicate links

[etherbob]

If rubygems/rubygems.org#3755 gets merged duplicate links in the gemspec will get hidden on rubygems.org and only the first in the list defined in models/links.rb will be displayed. @simi pointed out we could raise a warning to gem authors at build time by adding a warning to specification_policy.rb

I thought I would check in here and see if that's a desired feature before I dug too far into implementation.