If you're managing a project with many users, especially a public-facing project, you're probably used to users creating issues before checking for any similar issues that are already open or issues that may appear unrelated but have a similar cause. Thankfully, GitLab's Starter/Bronze level of enterprise subscription contains related issues:
With a Starter/Bronze enterprise subscription (or higher), you'll now see the Related issues box under the issue text. Click the plus symbol at the end and you'll be faced with a box where you can enter in related issues:
You can either paste a link to the issue or enter the issue ID preceded ...