Error Duplicate Column

rake aborted!
An error has occurred, this and all later migrations canceled:

PG::Error: ERROR:  column "created_at" of relation "posts" already exists
: ALTER TABLE "posts" ADD COLUMN "created_at" timestamp/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/connection_adapters/postgresql/database_statements.rb:128:in `exec'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/connection_adapters/postgresql/database_statements.rb:128:in `block in execute'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/connection_adapters/abstract_adapter.rb:425:in `block in log'
/app/vendor/bundle/ruby/2.0.0/gems/activesupport-4.0.0/lib/active_support/notifications/instrumenter.rb:20:in `instrument'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/connection_adapters/abstract_adapter.rb:420:in `log'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/connection_adapters/postgresql/database_statements.rb:127:in `execute'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/connection_adapters/abstract/schema_statements.rb:360:in `add_column'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/connection_adapters/postgresql/schema_statements.rb:379:in `add_column'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:625:in `block in method_missing'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:597:in `block in say_with_time'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:597:in `say_with_time'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:617:in `method_missing'
/app/db/migrate/20140507003956_add_time_stamp_to_post.rb:3:in `change'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:571:in `exec_migration'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:555:in `block (2 levels) in migrate'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:554:in `block in migrate'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/connection_adapters/abstract/connection_pool.rb:294:in `with_connection'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:553:in `migrate'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:709:in `migrate'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:959:in `block in execute_migration_in_transaction'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:1005:in `block in ddl_transaction'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/connection_adapters/abstract/database_statements.rb:202:in `block in transaction'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/connection_adapters/abstract/database_statements.rb:210:in `within_new_transaction'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/connection_adapters/abstract/database_statements.rb:202:in `transaction'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/transactions.rb:209:in `transaction'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:1005:in `ddl_transaction'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:958:in `execute_migration_in_transaction'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:920:in `block in migrate'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:916:in `each'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:916:in `migrate'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:764:in `up'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/migration.rb:742:in `migrate'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.0/lib/active_record/railties/databases.rake:42:in `block (2 levels) in <top (required)>'
Tasks: TOP => db:migrate
(See full trace by running task with --trace)