Fix Duo seed script for self-managed mode

What does this MR do and why?

• This rake task is used for seeding data/licenses for Duo features locally
• Previously, in self-managed mode this would fail if the group already existed.

References

• gitlab-development-kit#2767 (closed)

How to set up and validate locally

1. Run GITLAB_SIMULATE_SAAS=0 bundle exec rake gitlab:duo:setup twice, it should succeed both times

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