There should always be a single person assigned to an issue who carries responsibility for the issue. Of course, more than 1 person can contribute to an issue.

Commits can only be added to the master branch by means of a merge request. (Pushing to master is disabled.)

The development flow ideally goes like this:

1. Developer A is assigned to issue A.
2. Developer A creates a branch for issue A by clicking the button in the issue.
3. Developer A works in the created branch to address/solve the issue, and pushes the new commits to the repository.
4. Developer A opens a pull request for their branch to be merged to master, and assigns developer B to it.
5. Developer B reviews the changes, and maybe requests changes in the merge request.
6. When all issues with the merge requests are resolved, developer B merges it to master (by clicking the button).

When merge conflicts arise on a shared branch, we resolve them with git merge. (Discussion)