If you see this error:

Warning: this Gemfile contains multiple primary sources. Using `source` more than once without a block is a security risk, and may result in installing unexpected gems. To resolve this warning, use a block to indicate which gems should come from the secondary source. To upgrade this warning to an error, run `bundle config disable_multisource true`.

It means that you have more than one source in your Gemfile. For example if you use Rails Assets, it might look like this:

source 'https://rubygems.org'
source 'https://rails-assets.org'

# Gems list here...

It is unsafe, because the second source could “inject” a different version of your gems.

To prevent this, you need to declare which gems you want from the second source:

source 'https://rails-assets.org' do
  %w(
    jquery jquery-ujs bootstrap jquery-icheck select2 zeroclipboard
    font-awesome modernizer dropzone seiyria-bootstrap-slider jquery-masonry
    jquery-infinite-scroll imagesloaded markitup livestampjs datetimepicker
    videojs jquery.lazyload magnific-popup
  ).each do |asset_source|
    gem "rails-assets-#{asset_source}"
  end
end