diff --git a/CHANGES.rst b/CHANGES.rst
index 0232573..90ba967 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -5,7 +5,14 @@ Changelog
 2.0.5 (unreleased)
 ------------------
 
-* Add ``subpath`` option to specify a subdirectory of a repository. [mamico]
+- Allow a later source option to override an earlier one instead of raising
+  ``ValueError: Key '...' already in source info.``.  This lets a ``[sources]``
+  entry refine a shared/extended definition, e.g. ``foo += branch=my-feature``
+  overriding the ``branch`` set upstream.  Fixes `#125
+  <https://github.com/fschulze/mr.developer/issues/125>`_.
+  [janjaapdriessen]
+
+-  Add ``subpath`` option to specify a subdirectory of a repository. [mamico]
 
 
 2.0.4 (2025-07-17)
diff --git a/src/mr/developer/extension.py b/src/mr/developer/extension.py
index d6ce108..21051ec 100644
--- a/src/mr/developer/extension.py
+++ b/src/mr/developer/extension.py
@@ -115,7 +115,11 @@ def get_sources(self):
                 if not key:
                     raise ValueError("Option with no name '%s'." % option)
                 if key in source:
-                    raise ValueError("Key '%s' already in source info." % key)
+                    # A later option overrides an earlier one, e.g.
+                    # ``foo += branch=my-feature`` refining the ``branch`` set
+                    # in a shared/extended source definition.
+                    logger.info(
+                        "Overriding '%s' for source '%s'." % (key, name))
                 if key == 'path':
                     value = os.path.join(value, name)
                     if not os.path.isabs(value):
diff --git a/src/mr/developer/tests/test_extension.py b/src/mr/developer/tests/test_extension.py
index 0ffed5b..bbd0870 100644
--- a/src/mr/developer/tests/test_extension.py
+++ b/src/mr/developer/tests/test_extension.py
@@ -218,15 +218,16 @@ def testOptionParsingBeforeURL(self, buildout, extension):
         assert sources['pkg.foo']['rev'] == '>=456ad138'
 
     def testDuplicateOptionParsing(self, buildout, extension):
+        # A duplicate option is not an error: the later value wins, so a
+        # ``+=`` addition can override a value from a shared source definition
+        # (e.g. ``pkg.foo += branch=feature``).
         buildout['sources'].update({
             'pkg.foo': 'git dummy://foo/trunk rev=456ad138 rev=blubber',
+            'pkg.bar': 'git dummy://bar branch=main branch=feature',
         })
-        pytest.raises(ValueError, extension.get_sources)
-
-        buildout['sources'].update({
-            'pkg.foo': 'git dummy://foo/trunk kind=svn',
-        })
-        pytest.raises(ValueError, extension.get_sources)
+        sources = extension.get_sources()
+        assert sources['pkg.foo']['rev'] == 'blubber'
+        assert sources['pkg.bar']['branch'] == 'feature'
 
     def testInvalidOptionParsing(self, buildout, extension):
         buildout['sources'].update({