Gitlab fails to show there is a merge conflict for a open MR

Summary

Gitlab fails to show there is a merge conflict for a open MR.

Steps to reproduce

Possibly merge two branches at the same time

What is the current bug behavior?

In the UI it does not display there is a merge conflict

It may have been a race condition if two MRs were merged at the same time (which may have broken the UI's state).

What is the expected correct behavior?

Tell me there is a merge conflict

If it failed the race, it should have properly notified that the merge request failed due to a merge conflict.

Relevant logs and/or screenshots

Results of GitLab environment info

Gitlab EE 11.8.9 self hosted

root@git:/# gitlab-rake gitlab:env:info

System information
System:
Proxy:		no
Current User:	git
Using RVM:	no
Ruby Version:	2.5.3p105
Gem Version:	2.7.6
Bundler Version:1.16.6
Rake Version:	12.3.2
Redis Version:	3.2.12
Git Version:	2.18.1
Sidekiq Version:5.2.5
Go Version:	unknown

GitLab information
Version:	11.8.9-ee
Revision:	a00a46b
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	postgresql
DB Version:	9.6.11
URL:		
HTTP Clone URL:	
SSH Clone URL:	
Elasticsearch:	no
Geo:		no
Using LDAP:	yes
Using Omniauth:	yes
Omniauth Providers:

GitLab Shell
Version:	8.4.4
Repository storage paths:
- default: 	/var/opt/gitlab/git-data/repositories
Hooks:		/opt/gitlab/embedded/service/gitlab-shell/hooks
Git:		/opt/gitlab/embedded/bin/git
root@git:/#