diff --git a/conans/client/downloaders/download_cache.py b/conans/client/downloaders/download_cache.py
--- a/conans/client/downloaders/download_cache.py
+++ b/conans/client/downloaders/download_cache.py
@@ -60,9 +60,12 @@ def should_upload_sources(package):
         if excluded_urls is None:
             excluded_urls = []
 
-        all_refs = {str(k) for k, ref in package_list.refs()
-                    if ref.get("upload") or any(should_upload_sources(p)
-                                                for p in ref["packages"].values())}
+        all_refs = set()
+        for k, ref in package_list.refs():
+            packages = ref.get("packages", {}).values()
+            if ref.get("upload") or any(should_upload_sources(p) for p in packages):
+                all_refs.add(str(k))
+
         for f in os.listdir(path_backups):
             if f.endswith(".json"):
                 f = os.path.join(path_backups, f)

diff --git a/conans/test/integration/cache/backup_sources_test.py b/conans/test/integration/cache/backup_sources_test.py
--- a/conans/test/integration/cache/backup_sources_test.py
+++ b/conans/test/integration/cache/backup_sources_test.py
@@ -687,6 +687,7 @@ def source(self):
     def test_export_then_upload_workflow(self):
         client = TestClient(default_server_user=True)
         download_cache_folder = temp_folder()
+        mkdir(os.path.join(download_cache_folder, "s"))
         http_server = StoppableThreadBottle()
 
         http_server_base_folder_internet = temp_folder()
@@ -735,3 +736,56 @@ def source(self):
         client.run("create .")
         client.run("upload * -c -r=default")
         assert sha256 in os.listdir(http_server_base_folder_backup1)
+
+    def test_export_then_upload_recipe_only_workflow(self):
+        client = TestClient(default_server_user=True)
+        download_cache_folder = temp_folder()
+        mkdir(os.path.join(download_cache_folder, "s"))
+        http_server = StoppableThreadBottle()
+
+        http_server_base_folder_internet = temp_folder()
+        http_server_base_folder_backup1 = temp_folder()
+
+        sha256 = "315f5bdb76d078c43b8ac0064e4a0164612b1fce77c869345bfc94c75894edd3"
+        save(os.path.join(http_server_base_folder_internet, "myfile.txt"), "Hello, world!")
+
+        @http_server.server.get("/internet/<file>")
+        def get_internet_file(file):
+            return static_file(file, http_server_base_folder_internet)
+
+        @http_server.server.get("/downloader1/<file>")
+        def get_file(file):
+            return static_file(file, http_server_base_folder_backup1)
+
+        @http_server.server.put("/uploader/<file>")
+        def put_file(file):
+            dest = os.path.join(http_server_base_folder_backup1, file)
+            with open(dest, 'wb') as f:
+                f.write(request.body.read())
+
+        http_server.run_server()
+
+        conanfile = textwrap.dedent(f"""
+           from conan import ConanFile
+           from conan.tools.files import download
+           class Pkg2(ConanFile):
+               name = "pkg"
+               version = "1.0"
+               def source(self):
+                   download(self, "http://localhost:{http_server.port}/internet/myfile.txt", "myfile.txt",
+                            sha256="{sha256}")
+           """)
+
+        client.save({"global.conf": f"core.sources:download_cache={download_cache_folder}\n"
+                                    f"core.sources:download_urls=['http://localhost:{http_server.port}/downloader1/', 'origin']\n"
+                                    f"core.sources:upload_url=http://localhost:{http_server.port}/uploader/"},
+                    path=client.cache.cache_folder)
+
+        client.save({"conanfile.py": conanfile})
+        client.run("export .")
+        exported_rev = client.exported_recipe_revision()
+        client.run("upload * --only-recipe -c -r=default")
+        # This second run used to crash because we thought there would be some packages always
+        client.run("upload * --only-recipe -c -r=default")
+        # Ensure we are testing for an already uploaded recipe
+        assert f"Recipe 'pkg/1.0#{exported_rev}' already in server, skipping upload" in client.out

[bug] Can't call `conan upload --recipe-only` twice with backup sources enabled
### Steps to reproduce

1. Enable backup sources
2. Export a recipe that downloads file
3. Call conan upload only recipe for ref
4. Do it again, it fails due to KeyError


Found while prepping for https://github.com/conan-io/conan-center-index/pull/18082

conans/test/integration/cache/backup_sources_test.py::TestDownloadCacheBackupSources::test_export_then_upload_recipe_only_workflow
conans/test/integration/cache/backup_sources_test.py::TestDownloadCacheBackupSources::test_list_urls_miss conans/test/integration/cache/backup_sources_test.py::TestDownloadCacheBackupSources::test_ok_when_origin_authorization_error conans/test/integration/cache/backup_sources_test.py::TestDownloadCacheBackupSources::test_download_sources_multiurl conans/test/integration/cache/backup_sources_test.py::TestDownloadCacheBackupSources::test_ok_when_origin_breaks_midway_list conans/test/integration/cache/backup_sources_test.py::TestDownloadCacheBackupSources::test_upload_sources_backup conans/test/integration/cache/backup_sources_test.py::TestDownloadCacheBackupSources::test_export_then_upload_workflow conans/test/integration/cache/backup_sources_test.py::TestDownloadCacheBackupSources::test_download_origin_last conans/test/integration/cache/backup_sources_test.py::TestDownloadCacheBackupSources::test_sources_backup_server_error_500 conans/test/integration/cache/backup_sources_test.py::TestDownloadCacheBackupSources::test_users_download_cache_summary conans/test/integration/cache/backup_sources_test.py::TestDownloadCacheBackupSources::test_upload_sources_backup_creds_needed conans/test/integration/cache/backup_sources_test.py::TestDownloadCacheBackupSources::test_ok_when_origin_bad_sha256 conans/test/integration/cache/backup_sources_test.py::TestDownloadCacheBackupSources::test_download_origin_first