In the Gitlab settings Show archive.org snapshot the flag Auto-cancel redundant pipelines is enabled by default. This auto-cancels jobs that have the interruptible setting set to true (defaults to false e.g. to not cancel deploys by accident).

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Consider to set the interruptible flag for test jobs to reduce the load on your runners like in the following example .gitlab-ci.yml:

rubocop:
  interruptible: true
  script:
    - 'bundle exec rubocop'

rspec:
  interruptible: true
  script:
    - 'bundle exec rspec'

Afterwards new pushes to your MR will cancel all running interruptible jobs the previous pipelines and start a new pipeline with the current commit.

Tip

Instead of configuring jobs to be interruptible by default, you can also setup your .gitlab-ci.yml so commit messages with a string like no-ci don't start a new CI pipeline.