Pipeline triggers when it shouldn't

Drone Support
1

Here’s my drone.yaml, I want pipeline to run on pushes to master

kind: pipeline
type: docker
name: default

steps:
  - name: build
    image: docker:dind
    volumes:
      - name: dockersock
        path: /var/run
    commands:
      - echo hello

trigger:
  branch:
    - master
  event:
    - push

However it runs both when i create a PR for this branch and then merge it (thus pushing to master), resulting in two pipeline runs instead of one. What’s wrong with my config?