diff --git a/RELEASE_NOTES.md b/RELEASE_NOTES.md
index 62cacc3..7893425 100644
--- a/RELEASE_NOTES.md
+++ b/RELEASE_NOTES.md
@@ -2,13 +2,16 @@
 
 ## Summary
 
-This is the initial release of the Frequenz Core Library, which provides a set of fundamental tools and utilities for Python.
+<!-- Here goes a general summary of what this release is about -->
 
-The library currently includes:
+## Upgrading
 
-- `datetime`: For utilities related to dates and times.
-* `id`: For creating unique system-wide ID types.
-* `math`: For utilities related to math.
-* `typing`: For type annotations and type-checking utilities.
+<!-- Here goes notes on how to upgrade from previous versions, including deprecations and what they should be replaced with -->
 
-But more tools will be added in the future.
+## New Features
+
+<!-- Here goes the main new features and examples or instructions on how to use them -->
+
+## Bug Fixes
+
+* `BaseId` will now warn instead of raising an exception when a duplicate prefix is detected. This is to fix [a problem with code examples](https://github.com/frequenz-floss/frequenz-repo-config-python/issues/421) being tested using sybil and the class being imported multiple times, which caused the exception to be raised.
diff --git a/src/frequenz/core/id.py b/src/frequenz/core/id.py
index 2010948..80bfba2 100644
--- a/src/frequenz/core/id.py
+++ b/src/frequenz/core/id.py
@@ -63,6 +63,7 @@ class CustomNameForId(BaseId, str_prefix="CST", allow_custom_name=True):
 
 
 from typing import Any, ClassVar, Self, cast
+from warnings import warn
 
 
 class BaseId:
@@ -105,17 +106,18 @@ def __init_subclass__(
             **kwargs: Forwarded to the parent's __init_subclass__.
 
         Raises:
-            ValueError: If the `str_prefix` is already registered by another
-                ID type.
             TypeError: If `allow_custom_name` is False and the class name
                 does not end with "Id".
         """
         super().__init_subclass__(**kwargs)
 
         if str_prefix in BaseId._registered_prefixes:
-            raise ValueError(
+            # We want to raise an exception here, but currently can't due to
+            # https://github.com/frequenz-floss/frequenz-repo-config-python/issues/421
+            warn(
                 f"Prefix '{str_prefix}' is already registered. "
-                "ID prefixes must be unique."
+                "ID prefixes must be unique.",
+                stacklevel=2,
             )
         BaseId._registered_prefixes.add(str_prefix)
 
diff --git a/tests/test_id.py b/tests/test_id.py
index 30916b9..a751348 100644
--- a/tests/test_id.py
+++ b/tests/test_id.py
@@ -26,6 +26,24 @@ def test_valid() -> None:
     assert int(id_obj) == 42
 
 
+def test_use_a_subclass() -> None:
+    """Test that BaseId cannot be instantiated directly."""
+    with pytest.raises(
+        TypeError, match="BaseId cannot be instantiated directly. Use a subclass."
+    ):
+        BaseId(42)
+
+
+def test_warn_non_unique_prefix() -> None:
+    """Test that using a non-unique prefix raises a warning."""
+    with pytest.warns(UserWarning, match="Prefix 'TEST_ID' is already registered"):
+
+        class _TestDuplicateId(BaseId, str_prefix="TEST_ID"):
+            """A duplicate test ID class with the same prefix as _TestId."""
+
+        _TestDuplicateId(1)
+
+
 def test_negative_raises() -> None:
     """Test that creating a negative ID raises ValueError."""
     with pytest.raises(ValueError, match="_TestId can't be negative"):