diff --git a/git_hg_sync/config.py b/git_hg_sync/config.py
index abe7bea..4fc6141 100644
--- a/git_hg_sync/config.py
+++ b/git_hg_sync/config.py
@@ -1,4 +1,5 @@
 import pathlib
+from collections import Counter
 from typing import Annotated, Self, override
 
 import tomllib
@@ -81,6 +82,18 @@ def verify_all_mappings_reference_tracked_repositories(
                 )
         return self
 
+    @model_validator(mode="after")
+    def verify_unique_tracked_repositories_name(self) -> Self:
+        tracked_names = Counter(
+            tracked_repo.name for tracked_repo in self.tracked_repositories
+        )
+        non_unique = [name for name in tracked_names if tracked_names[name] > 1]
+        if non_unique:
+            raise ValueError(
+                f"Found non-unique names in tracked_repositories: {', '.join(non_unique)}"
+            )
+        return self
+
     @override
     @classmethod
     def settings_customise_sources(
diff --git a/tests/test_config.py b/tests/test_config.py
index 5a1c6c4..18f0af0 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -55,6 +55,18 @@ def test_load_config_env_override(monkeypatch: pytest.MonkeyPatch) -> None:
             )
 
 
+def test_unique_tracked_repository_name() -> None:
+    config = tomllib.loads((HERE / "data" / "config.toml").read_text())
+
+    # Ensure this doesn't fail without duplicates.
+    _ = Config(**config)
+
+    config["tracked_repositories"].append(config["tracked_repositories"][0])
+
+    with pytest.raises(ValueError, match="non-unique names"):
+        _ = Config(**config)
+
+
 @pytest.mark.parametrize(
     "source_url,source_branch,expected_urls,expected_branches",
     [
diff --git a/tests/test_config_files.py b/tests/test_config_files.py
index cf03aea..318f340 100644
--- a/tests/test_config_files.py
+++ b/tests/test_config_files.py
@@ -24,7 +24,7 @@ def test_config_files(monkeypatch: pytest.MonkeyPatch, config_file: Path) -> Non
     try:
         config = Config.from_file(config_file)
     except ValidationError as exc:
-        raise AssertionError(f"Syntax error in {config_file}") from exc
+        raise AssertionError(f"Syntax error in {config_file}: {exc}") from exc
 
     # We just do a shallow verification. What we really care is that the file could be
     # loaded correctly.