Rails SQLite 3 duplicate column error

I'm trying to install Active Admin, which is a Ruby on Rails framework for administrator dashboard for websites. However everything went fine in installation when trying to rake db:migrate I get the following error.

rake db:migrate 

SQLite3::SQLException: duplicate column name: email: ALTER TABLE "admin_users" ADD "email" varchar DEFAULT '' NOT NULL/usr/local/rvm/gems/ruby-2.2.1/gems/sqlite3-1.3.10/lib/sqlite3/database.rb:91:in `initialize'
/usr/local/rvm/gems/ruby-2.2.1/gems/sqlite3-1.3.10/lib/sqlite3/database.rb:91:in `new'
/usr/local/rvm/gems/ruby-2.2.1/gems/sqlite3-1.3.10/lib/sqlite3/database.rb:91:in `prepare'
/usr/local/rvm/gems/ruby-2.2.1/gems/sqlite3-1.3.10/lib/sqlite3/database.rb:134:in `execute'
/usr/local/rvm/gems/ruby-2.2.1/gems/activerecord-4.2.1/lib/active_record/connection_adapters/sqlite3_adapter.rb:329:in `block in execute'
/usr/local/rvm/gems/ruby-2.2.1/gems/activerecord-4.2.1/lib/active_record/connection_adapters/abstract_adapter.rb:473:in `block in log'
/usr/local/rvm/gems/ruby-2.2.1/gems/activesupport-4.2.1/lib/active_support/notifications/instrumenter.rb:20:in `instrument'
/usr/local/rvm/gems/ruby-2.2.1/gems/activerecord-4.2.1/lib/active_record/connection_adapters/abstract_adapter.rb:467:in `log'
........................

How can I fix this? Thanks