RecordNotUnique error when using Data Seeder

Problem

While running the Data Seeder, a frequent error that is encountered is:

Execute ee:gitlab:seed:data_seeder rake aborted!

ActiveRecord::RecordNotUnique: PG::UniqueViolation:

ERROR: duplicate key value violates unique constraint "index_ci_project_mirrors_on_project_id" DETAIL: Key (project_id)=(774206) already exists.

Stack Trace

root@192:/opt/gitlab/embedded/service/gitlab-rails# gitlab-rake "ee:gitlab:seed:data_seeder[bulk_data.rb]"
Seeding data for Administrator
.rake aborted!
ActiveRecord::RecordNotUnique: PG::UniqueViolation: ERROR:  duplicate key value violates unique constraint "index_ci_namespace_mirrors_on_namespace_id"
DETAIL:  Key (namespace_id)=(2) already exists.
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/load_balancing/connection_proxy.rb:127:in `public_send'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/load_balancing/connection_proxy.rb:127:in `block in write_using_load_balancer'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/load_balancing/load_balancer.rb:133:in `block in read_write'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/load_balancing/load_balancer.rb:204:in `retry_with_backoff'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/load_balancing/load_balancer.rb:122:in `read_write'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/load_balancing/connection_proxy.rb:126:in `write_using_load_balancer'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/load_balancing/connection_proxy.rb:61:in `block (2 levels) in <class:ConnectionProxy>'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/load_balancing/connection_proxy.rb:127:in `public_send'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/load_balancing/connection_proxy.rb:127:in `block in write_using_load_balancer'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/load_balancing/load_balancer.rb:133:in `block in read_write'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/load_balancing/load_balancer.rb:204:in `retry_with_backoff'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/load_balancing/load_balancer.rb:122:in `read_write'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/load_balancing/connection_proxy.rb:126:in `write_using_load_balancer'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/load_balancing/connection_proxy.rb:78:in `transaction'
/opt/gitlab/embedded/service/gitlab-rails/app/models/concerns/cross_database_modification.rb:92:in `block in transaction'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/load_balancing/connection_proxy.rb:127:in `public_send'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/load_balancing/connection_proxy.rb:127:in `block in write_using_load_balancer'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/load_balancing/load_balancer.rb:133:in `block in read_write'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/load_balancing/load_balancer.rb:204:in `retry_with_backoff'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/load_balancing/load_balancer.rb:122:in `read_write'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/load_balancing/connection_proxy.rb:126:in `write_using_load_balancer'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database/load_balancing/connection_proxy.rb:78:in `transaction'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database.rb:390:in `block in transaction'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/database.rb:389:in `transaction'
/opt/gitlab/embedded/service/gitlab-rails/app/models/concerns/cross_database_modification.rb:83:in `transaction'
/opt/gitlab/embedded/service/gitlab-rails/spec/factories/namespaces.rb:19:in `block (3 levels) in <top (required)>'
<internal:kernel>:90:in `tap'

This error usually goes away on a re-run of the seeder.

Proposal

Look into this issue and fix it.