Issue Summary

When attempting to import a repository via Git, if you quickly click the "Import Repository" button twice, it will throw the following error:

Caused by: org.postgresql.util.PSQLException: ERROR: duplicate key value violates unique constraint "uk_slug_project_id"
  Detail: Key (slug, project_id)=(react, 26) already exists.
    at org.postgresql.core.v3.QueryExecutorImpl.receiveErrorResponse(QueryExecutorImpl.java:2713)
    at org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:2401)
    ... 92 common frames omitted 

This is reproducible on Data Center: yes

Steps to Reproduce

1. Open the "Import repositories" page in any given project 
2. Select "Import from" Git
3. Paste in the given URL (and any credentials if required)
4. Double click the "Import Repository" button 

Expected Results

The button should either disable after the first click or have no effect upon the second and the repository should be imported successfully with no errors.

Actual Results

The exception above is thrown and the UI shows an error (though, the repository is eventually imported).

Workaround

Ensure you only click the "Import Repository" button once.