add error for trying to sign up with the same oauth account twice

api/user.go

@@ -1277,7 +1277,7 @@ func AuthorizeOAuthUser(service, code, state, redirectUri string) (io.ReadCloser
 
 	var ar *model.AccessResponse
 	if resp, err := client.Do(req); err != nil {
-		return nil, model.NewAppError("AuthorizeOAuthUser", "Token request to GitLab failed", err.Error())
+		return nil, model.NewAppError("AuthorizeOAuthUser", "Token request failed", err.Error())
 	} else {
 		ar = model.AccessResponseFromJson(resp.Body)
 	}

web/web.go

@@ -499,6 +499,11 @@ func signupCompleteOAuth(c *api.Context, w http.ResponseWriter, r *http.Request)
 			return
 		}
 
+		if result := <-api.Srv.Store.User().GetByAuth(team.Id, user.AuthData, service); result.Err == nil {
+			c.Err = model.NewAppError("signupCompleteOAuth", "This "+service+" account has already been used to sign up for team "+team.DisplayName, "email="+user.Email)
+			return
+		}
+
 		if result := <-api.Srv.Store.User().GetByEmail(team.Id, user.Email); result.Err == nil {
 			c.Err = model.NewAppError("signupCompleteOAuth", "Team "+team.DisplayName+" already has a user with the email address attached to your "+service+" account", "email="+user.Email)
 			return