I'm getting an ActiveRecord::DuplicateMigrationNameError during rake db:migrate

My steps are:

• Add to Gemfile:

gem 'kaminari' # required by coupons
gem 'coupons', :git => 'https://github.com/fnando/coupons.git'

• bundle install
• rake coupons:install:migrations
• rake db:migrate which generates the following error:

rake aborted!
ActiveRecord::DuplicateMigrationNameError: 

Multiple migrations have the name SetupCoupons

I only have new 1 migration file in db/migrate and none of my existing migrations have the name SetupCoupons.

Rails 4.2.6
Ruby 2.3.1p112
Bundler 1.12.5