api returns 404 when creating duplicate merge request

Created by: burr86

Summary: Submitting an identical merge request returns a 404 the second time. Steps to reproduce: POST /projects/:project_id/merge_requests with the standard MR parameters Expected behavior: a more descriptive error message, or just returning the existing merge request. Observed behavior: It returns a 404. Output of checks: Version we're running is 6.1 ... I can include other details if you think they're relevant but I'm pretty sure this isn't a configuration issue. Checks all pass in my environment. Possible fixes: I don't know enough to say. :D