The build job is executed for every OS, including the creation of a GitHub release. You should only create the release once, and then let each build job upload the release assets to the same release. Maybe like this:

name: Create Release Builds
on:

push:

tags:

- "v*" # matches v1.0.1, v1.2, v2, etc
jobs:

once:

name: Create GitHub release

runs-on: ubuntu-latest

outputs:

upload_url: ${{ steps.create_release.outputs.upload_url }}

steps:

- name: Create a release

id: create_release

uses: actions/create-release@v1

env:

GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

with:

tag_name: ${{ github.ref }}

release_name: Release ${{ github.ref }}

draft: true

prerelease: true
build:

name: Create cross-platform release build, tag and upload binaries

needs: once

runs-on: ${{ matrix.os }}

strategy:

matrix:

os: [ubuntu-latest, macos-latest, windows-latest]

steps:

- uses: actions/checkout@v1

with:

submodules: true
  - name: Install latest CMake
    uses: lukka/get-cmake@latest

  - name: Run CMake (generation and build)
    uses: lukka/run-cmake@v2
    with:
      cmakeListsOrSettingsJson: "CMakeListsTxtBasic"
      cmakeBuildType: Release
      buildDirectory: "${{ github.workspace }}/${{ matrix.os }}"

  - name: Upload Release Asset
    id: upload-release-asset
    uses: actions/upload-release-asset@v1
    env:
      GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
    with:
      upload_url: ${{ needs.once.outputs.upload_url }}
      asset_path: ${{ github.workspace }}/${{ matrix.os }}/xtest
      asset_name: xtest-${{ matrix.os }}
      asset_content_type: application/octet-stream