test-infra: hook: blunderbuss: do not attempt to assign someone who's already assigned

{
    "author": "saschagrunert",
    "component": "hook",
    "error": "status code 422 not one of [201], body: {\"message\":\"Validation Failed\",\"errors\":[\"Could not add assignees: Validation failed: Assignee has already been taken\"],\"documentation_url\":\"https://developer.github.com/v3/issues/assignees/#add-assignees-to-an-issue\"}",
    "event-GUID": "08b60c80-4830-11ea-8fea-d93e76628065",
    "event-type": "issue_comment",
    "file": "prow/plugins/lgtm/lgtm.go:294",
    "func": "k8s.io/test-infra/prow/plugins/lgtm.handle",
    "level": "error",
    "msg": "Failed to assign cri-o/cri-o#3201 to saschagrunert",
    "org": "cri-o",
    "plugin": "lgtm",
    "pr": 3201,
    "repo": "cri-o",
    "time": "2020-02-05T15:56:15Z",
    "url": "https://github.com/cri-o/cri-o/pull/3201#issuecomment-582475910"
}