[Bug]: generate_release_notes: true duplicates release notes when release already exists on re-run #827

Pre-flight checks

• I searched existing issues and did not find a duplicate
• I reproduced this with the latest released version or current master
• I included a reproducible example or a sanitized/redacted reproduction path if the original repository is private

action-gh-release version

v3.0.2

Runner operating system

macos-latest

Release target repository

No response

Reproduction repo, gist, or artifact

No response

Workflow snippet

- name: Create GitHub Release
  uses: softprops/action-gh-release@v3
  with:
    tag_name: "1.0.0-beta"
    prerelease: true
    generate_release_notes: true
    previous_tag: "1.0.0-prev"
    append_body: false

Expected behavior

When a release already exists for the given tag, generate_release_notes should
overwrite the existing body with freshly generated notes — not merge/append them.

Actual behavior

On every re-run, the generated notes are merged into the existing release body,
causing the notes to appear duplicated. Setting append_body: false does not
prevent this because append_body only controls manually provided body/body_path
inputs — it has no effect on generate_release_notes behaviour.

Steps to reproduce

1. Run the workflow for a tag — release is created with notes successfully
2. Re-run the exact same workflow for the same tag (e.g. deploying a second flavour)
3. Observe that release notes are duplicated in the existing release body

Relevant logs

No error logs — the action succeeds but the release body contains duplicated notes

Additional context

Current workaround: manually clear the release body before the action runs using gh release edit "$TAG" --notes "", then let the action regenerate notes fresh. But this is a workaround and ideally the action should handle
this natively.