Issues appear on multiple pages in issue list when sorted on weight

Summary

When browsing through a project's issue list, and sorting by weight, an item can appear on page 1 AND page 2, or page 2 AND page 3. This is scary, because it means some issues are completely absent from the list.

Steps to reproduce

Create 41 issues (so that you get 3 pages of issues), all with the same weight. Go to page 1, copy all the issue numbers (for example by the following javascript snippet: $(".issue").each(function (x) { console.log($(this).data('id'));});, do the same for pages 2 and 3. You will see that there are duplicates.

What is the current bug behavior?

An issues can appear on multiple pages when paginating while ordered on weight.

What is the expected correct behavior?

An issue should only appear on 1 page.

Relevant logs and/or screenshots

See the following screenshots where issues 6, 8, 9 and 10 appear on multiple pages.

Results of GitLab environment info

System information
System:		Debian 8.7
Proxy:		no
Current User:	gitlab
Using RVM:	no
Ruby Version:	2.3.3p222
Gem Version:	2.6.6
Bundler Version:1.13.7
Rake Version:	10.5.0
Redis Version:	3.2.5
Sidekiq Version:4.2.7

GitLab information
Version:	8.17.2-ee
Revision:	474978d
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	postgresql
DB Version:	9.2.18
URL:		https://gitlab.srv.hq.mendix.net
HTTP Clone URL:	https://gitlab.srv.hq.mendix.net/some-group/some-project.git
SSH Clone URL:	gitlab@gitlab.srv.hq.mendix.net:some-group/some-project.git
Elasticsearch:	no
Geo:		no
Using LDAP:	yes
Using Omniauth:	yes
Omniauth Providers: github

GitLab Shell
Version:	4.1.1
Repository storage paths:
- default: 	/var/opt/gitlab/git-data/repositories
Hooks:		/opt/gitlab/embedded/service/gitlab-shell/hooks/
Git:		/opt/gitlab/embedded/bin/git

Results of GitLab application Check

Checking GitLab Shell ...

GitLab Shell version >= 4.1.1 ? ... OK (4.1.1)
Repo base directory exists?
default... yes
Repo storage directories are symlinks?
default... no
Repo paths owned by gitlab:gitlab?
default... yes
Repo paths access is drwxrws---?
default... yes
hooks directories in repos are links: ... 
4/1 ... ok
...
4/26 ... repository is empty
...
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Check GitLab API access: OK
Access to /var/opt/gitlab/.ssh/authorized_keys: OK
Send ping to redis server: OK
gitlab-shell self-check successful

Checking GitLab Shell ... Finished

Checking Sidekiq ...

Running? ... yes
Number of Sidekiq processes ... 1

Checking Sidekiq ... Finished

Checking Reply by email ...

Reply by email is disabled in config/gitlab.yml

Checking Reply by email ... Finished

Checking LDAP ...

Server: ldapmain
LDAP authentication... Success
LDAP users with access to your GitLab server (only showing the first 100 results)
    ... [SNIP] ...

Checking LDAP ... Finished

Checking GitLab ...

Git configured with autocrlf=input? ... yes
Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config outdated? ... no
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory setup correctly? ... no
  Try fixing it:
  sudo chown -R gitlab /var/opt/gitlab/gitlab-rails/uploads
  sudo find /var/opt/gitlab/gitlab-rails/uploads -type f -exec chmod 0644 {} \;
  sudo find /var/opt/gitlab/gitlab-rails/uploads -type d -not -path /var/opt/gitlab/gitlab-rails/uploads -exec chmod 0700 {} \;
  For more information see:
  doc/install/installation.md in section "GitLab"
  Please fix the error above and rerun the checks.
Init script exists? ... skipped (omnibus-gitlab has no init script)
Init script up-to-date? ... skipped (omnibus-gitlab has no init script)
projects have namespace: ... 
    ... [SNIP] ...

Redis version >= 2.8.0? ... yes
Ruby version >= 2.1.0 ? ... yes (2.3.3)
Your git bin path is "/opt/gitlab/embedded/bin/git"
Git version >= 2.7.3 ? ... yes (2.10.2)
Active users: 123

Checking GitLab ... Finished

Possible fixes

n/a