[a.rb]
require 'b'
require 'c'
class A;X='';end

[b.rb]
require 'a'
require 'c'
class B;Y='';end

[c.rb]
require 'a'
require 'b'
class C;Z='';end

$ ruby a.rb
a.rb:3: warning: already initialized constant X

$ ruby b.rb
b.rb:3: warning: already initialized constant Y

$ ruby c.rb
c.rb:3: warning: already initialized constant Z