PR Merge does 2 way merge instead of one way

I hv 2 protected branches ‘dev-1’ & ‘common-dev’. Want 2 merge dev-1 into common-dev, I created a PR in github web (dev-1= head, common-dev=base). When PR is merged, it does a 2way merge, both branches get merged with each other. How to stop this? I need only dev1 to be merged to common-dev, not the other way around

Would creating a protected branch rule in the repository help?