Strange error using Rails

[sistrall]

Hello everyone,

I'm trying rack-cache on a Rails application, using it without problem
when developing locally.

After the deploy I'm going through an error (that I really not
understand). Here it is:

can't activate rack (>= 0.4, runtime) for ["rack-cache-0.5.2"],
already activated rack-1.0.0 for []

I dont' understand why there should be the need to activate rack when
rack is already active. This error is generated when the code requires
rack-cache.

These are the relevant gems installed on the system generating error
(I don't know if this could help):

rack (1.0.1, 1.0.0, 0.9.1, 0.4.0, 0.3.0)
rack-cache (0.5.2)
rails (2.3.4, 2.3.1, 2.2.2, 2.1.1, 2.1.0, 0.14.3)

Any help is welcome,
thank you
Silvano