diff --git a/src/ModuleCore/Git/GitManager.cs b/src/ModuleCore/Git/GitManager.cs
index 0d5af37..25a5b41 100644
--- a/src/ModuleCore/Git/GitManager.cs
+++ b/src/ModuleCore/Git/GitManager.cs
@@ -138,8 +138,15 @@ public class GitManager
 			// we add it to the relevant dictionary
 			if (directorySegmentsFromName.Count == 0)
 			{
-				FullRepositoryPath = absoluteRepositoryLocation;
-				return this;
+				// If this is a new registration, set the FullRepositoryPath and return, otherwise we've got a duplicate
+				// entry and we throw
+				if (FullRepositoryPath == null)
+				{
+					FullRepositoryPath = absoluteRepositoryLocation;
+					return this;
+				}
+
+				throw new Exception($"Registration already exists for {InternalPath}");
 			}
 
 			var nextSegment = directorySegmentsFromName.Peek();
diff --git a/tests/ModuleTests/Git/AddRegistrationTests.cs b/tests/ModuleTests/Git/AddRegistrationTests.cs
index 6de31d6..e04cfc3 100644
--- a/tests/ModuleTests/Git/AddRegistrationTests.cs
+++ b/tests/ModuleTests/Git/AddRegistrationTests.cs
@@ -74,4 +74,23 @@ public class AddRegistrationTests
 
 		Assert.Equal("repo", whitespaceName);
 	}
+
+	[Fact]
+	public void DuplicateRepoRegistrationShouldFail()
+	{
+		Settings.UseFileName(nameof(RepoRegistrationWithWhitespaceName));
+
+		var gitManager = GitManager.InternalFreshInstance;
+		var testRepoAbsolutePath = "Test:/some/test/repo";
+		string[] paths = ["test", "nested", "path"];
+		var names = (NormalSeparator: string.Join(Path.DirectorySeparatorChar, paths), AltSeparator: string.Join(Path.AltDirectorySeparatorChar, paths));
+
+		var firstRegistration = gitManager.RegisterRepo(testRepoAbsolutePath, names.NormalSeparator);
+		// TODO: make nested registrations fail in both directions and test
+		var secondRegistration = gitManager.RegisterRepo(testRepoAbsolutePath, Path.Combine(paths[..1]));
+
+		Assert.Equal(Path.Combine(paths), firstRegistration);
+
+		Assert.Throws<Exception>(() => gitManager.RegisterRepo(testRepoAbsolutePath, names.AltSeparator));
+	}
 }
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