Not multiple times per request, but rather reloaded per request. The first request may be fine, but subsequent requests will show that error. A constant is exactly that: a constant variable for the life of your application. If you're defining constants in your application_helper, maybe you should be doing it somewhere else?
But this is not the case with application_helper.rb, in the sense that ApplicationHelper is autoloaded:
Loading development environment (Rails 2.0.2)
>> Dependencies.autoloaded? ApplicationHelper
That is a side-effect of prepare_application.
So, you know, you get a fresh new module in the fresh new constant ApplicationHelper per request in development mode. Any nested constant is new in consequence, hence if everything is set up as usual you don't get those warnings.