Ruby: Constant values

I generally use constants in Ruby for the following two situations: Markers or Constant values.

Markers are used as a standard for comparison.

module CreditCardTypes
  Visa = 0
  Mastercard = 1
end

Markers are initialized to a value; however, that value is unimportant as long as it is unique. Markers are generally used in an application within conditional statements.

case card.type
when CreditCardTypes::Visa then VisaLuhnValidator
when CreditCardTypes::Mastercard then MastercardLuhnValidator
end

Note: you can also use symbols as markers, but I prefer constants. This preference is based on the fact that if I mistype CreditCardTypes::Vissa it will fail fast; however, if I mistype :credit_card_type_vissa, I will get a possibly hard to find bug.

Constant values are global values that should never change during the life of your application.

module MathValues
  PI = 3.14
end

Constant values can be used throughout applications to ensure that the same value is consistently used.

circumference = circle.diameter * MathValues::PI

Based on these usages, I'm a bit concerned about some behavior I recently found.

irb(main):019:0> module MathVariables
irb(main):020:1>   PI = 3.14
irb(main):021:1> end
=> 3.14
irb(main):022:0> module MathVariables
irb(main):023:1>   PI = 3.14159265
irb(main):024:1> end
(irb):23: warning: already initialized constant PI
=> 3.14159265

Warning? This means anyone can redefine my constants at any time? Did I do something wrong? Does anyone else think this is dangerous?