You have already activated rack (gem name) 1.4.1(gem version), but your Gemfile requires rack (gem name) 1.3.6 (gem version)

Error message:
You have already activated rack (gem name) 1.4.1(gem version), but your Gemfile requires rack (gem name) 1.3.6 (gem version)

This could be any gem and any version, this is just my example using the rack gem.


Problem:
On my production machine I am using passenger 3.0.13 and it requires rack 1.4.1. In my Gemfile I am using Rails 3.1.3. Unfortunately rails 3.1.3 uses actionpack (and others) which depends on rack 1.3.6. I am not using rvm so rack 1.4.1 loads when passenger loads and therefore is loaded before rack 1.3.6. Since you can't have more than one version running, I get the error. Makes since, right?


Fix:
There are multiple possible fixes.
1. install rvm and add a gemset
2. (the one that I used) I decided to just upgrade rails to 3.2.6. This uses rack 1.4.1 and now all is happy :)
3. downgrade passenger to a version that uses rack 1.3.6