Search for @my_data and you’ll find the example about that.

By default you can overwrite the value of a module attribute at any time, which is totally valid. If instantiated correctly they can also be used to aggregate data, which is how most of plugs functionality is working.

It’s definitely valid, but what’s going on?.. it depends.

@param :foo
@param :bar     
# here @param == :bar

But an attribute can also be registered to accumulate the values:

defmodule Foo do
  Module.register_attribute __MODULE__, :param, accumulate: true

  @param :foo
  @param :bar     
  # here @param == [:foo, :bar]
end

If you’re useing a module/macro system, sometimes it’s hard to tell how exactly these attributes are used. @tag in ExUnit probably isn’t accumulating its values, but using them everytime you call the test macro:

defmodule SomeTest do
  use ExUnit.Case

  @tag :foo  
  test "first test" do
    # here the `test` macro fetches the current value of @tag, which at this moment is :foo
  end

  @tag :bar 
  test "first test" do
    # here @tag == :bar
  end
end

I’m not too sure ExUnit works like this, but the general gist of this, and hopefully to answer your question, is that attributes can be set multiple times, and how are they used exactly depends on who’s consuming them