fix sdist build for multiple readme files

src/poetry/core/masonry/builders/sdist.py

@@ -15,6 +15,7 @@
 from pprint import pformat
 from typing import TYPE_CHECKING
 from typing import Iterator
+from typing import Sequence
 
 from poetry.core.masonry.builders.builder import Builder
 from poetry.core.masonry.builders.builder import BuildIncludeFile
@@ -328,9 +329,13 @@ def find_files_to_add(self, exclude_build: bool = False) -> set[BuildIncludeFile
         # Include project files
         additional_files.add(Path("pyproject.toml"))
 
-        # add readme if it is specified
+        # add readme files if it is specified
         if "readme" in self._poetry.local_config:
-            additional_files.add(self._poetry.local_config["readme"])
+            r: str | Sequence = self._poetry.local_config["readme"]
+            if isinstance(r, str):
+                additional_files.add(r)
+            else:
+                additional_files.update(r)
 
         for additional_file in additional_files:
             file = BuildIncludeFile(

tests/masonry/builders/test_sdist.py

@@ -191,6 +191,24 @@ def test_find_files_to_add() -> None:
     )
 
 
+def test_find_files_to_add_with_multiple_readme_files() -> None:
+    poetry = Factory().create_poetry(
+        Path(__file__).parent.parent.parent / "fixtures" / "with_readme_files"
+    )
+
+    builder = SdistBuilder(poetry)
+    result = [f.relative_to_source_root() for f in builder.find_files_to_add()]
+
+    assert sorted(result) == sorted(
+        [
+            Path("README-1.rst"),
+            Path("README-2.rst"),
+            Path("my_package/__init__.py"),
+            Path("pyproject.toml"),
+        ]
+    )
+
+
 def test_make_pkg_info_multi_constraints_dependency() -> None:
     poetry = Factory().create_poetry(
         Path(__file__).parent.parent.parent