module Roda::RodaPlugins::ContentFor

def self.configure(app, opts = OPTS)

is called multiple times with the same key.
Configure whether to append or overwrite if content_for 
def self.configure(app, opts = OPTS)
  app.opts[:append_content_for] = opts.fetch(:append, true)
end

def self.load_dependencies(app, _opts = OPTS)

to capture the content.
Depend on the capture_erb plugin, since it uses capture_erb 
def self.load_dependencies(app, _opts = OPTS)
  app.plugin :capture_erb
end