diff --git a/src/CoreTests/Bugs/Bug_4185_codegen_conflict_projection_with_secondary_store_dependency.cs b/src/CoreTests/Bugs/Bug_4185_codegen_conflict_projection_with_secondary_store_dependency.cs
new file mode 100644
index 0000000000..5d5d25bcd8
--- /dev/null
+++ b/src/CoreTests/Bugs/Bug_4185_codegen_conflict_projection_with_secondary_store_dependency.cs
@@ -0,0 +1,223 @@
+using System;
+using System.IO;
+using System.Linq;
+using System.Threading.Tasks;
+using JasperFx.CodeGeneration;
+using JasperFx.Core;
+using JasperFx.Events.Projections;
+using JasperFx.RuntimeCompiler;
+using Marten;
+using Marten.Events.Aggregation;
+using Marten.Testing.Harness;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
+using Shouldly;
+using Xunit;
+
+namespace CoreTests.Bugs;
+
+public interface IBug4185Store : IDocumentStore { }
+public interface IBug4185OtherStore : IDocumentStore { }
+
+public record OrderPlaced4185(string ProductName, decimal UnitPrice, int Quantity);
+public record OrderShipped4185(DateTime ShippedAt);
+
+public class OrderSummary4185
+{
+    public Guid Id { get; set; }
+    public string ProductName { get; set; }
+    public decimal Total { get; set; }
+    public bool Shipped { get; set; }
+}
+
+/// <summary>
+/// Projection on the primary store that directly injects a secondary store.
+/// This is the pattern that causes the codegen conflict.
+/// </summary>
+public class OrderProjection4185 : SingleStreamProjection<OrderSummary4185, Guid>
+{
+    private readonly IBug4185Store _secondaryStore;
+
+    public OrderProjection4185(IBug4185Store secondaryStore)
+    {
+        _secondaryStore = secondaryStore;
+    }
+
+    public OrderSummary4185 Create(OrderPlaced4185 e)
+    {
+        return new OrderSummary4185
+        {
+            ProductName = e.ProductName,
+            Total = e.UnitPrice * e.Quantity
+        };
+    }
+
+    public void Apply(OrderShipped4185 e, OrderSummary4185 summary)
+    {
+        summary.Shipped = true;
+    }
+}
+
+/// <summary>
+/// Reproduces https://github.com/JasperFx/marten/issues/4185
+///
+/// When a secondary document store is registered, the codegen write command
+/// (DynamicCodeBuilder.WriteGeneratedCode) iterates all ICodeFileCollection
+/// instances. The SecondaryDocumentStores collection writes the store
+/// implementation to {base}/Stores/{StoreImpl}.cs.
+///
+/// Separately, at runtime, SecondaryStoreConfig.Build() calls
+/// InitializeSynchronously() with rules from CreateGenerationRules() which
+/// include the StoreName in the path, writing the same class to
+/// {base}/{StoreName}/Stores/{StoreImpl}.cs.
+///
+/// Both files share namespace Marten.Generated.Stores and the same class name,
+/// causing CS0101 duplicate type definition when using TypeLoadMode.Static.
+/// </summary>
+public class Bug_4185_codegen_conflict_projection_with_secondary_store_dependency
+{
+    /// <summary>
+    /// Simulates what "dotnet run -- codegen write" does by using
+    /// DynamicCodeBuilder.WriteGeneratedCode(), then verifies that
+    /// no duplicate store implementation files are produced.
+    /// </summary>
+    [Fact]
+    public async Task codegen_write_should_not_produce_duplicate_secondary_store_implementations()
+    {
+        var tempDir = Path.Combine(Path.GetTempPath(), "bug4185_" + Guid.NewGuid().ToString("N")[..8]);
+        Directory.CreateDirectory(tempDir);
+
+        try
+        {
+            using var host = await Host.CreateDefaultBuilder()
+                .ConfigureServices(services =>
+                {
+                    services.AddMartenStore<IBug4185Store>(opts =>
+                    {
+                        opts.Connection(ConnectionSource.ConnectionString);
+                        opts.DatabaseSchemaName = "bug4185_sec";
+                        opts.GeneratedCodeMode = TypeLoadMode.Auto;
+                        opts.GeneratedCodeOutputPath = tempDir;
+                    });
+
+                    services.AddMartenStore<IBug4185OtherStore>(opts =>
+                    {
+                        opts.Connection(ConnectionSource.ConnectionString);
+                        opts.DatabaseSchemaName = "bug4185_oth";
+                        opts.GeneratedCodeMode = TypeLoadMode.Auto;
+                        opts.GeneratedCodeOutputPath = tempDir;
+                    });
+
+                    services.AddMarten(opts =>
+                    {
+                        opts.Connection(ConnectionSource.ConnectionString);
+                        opts.DatabaseSchemaName = "bug4185_pri";
+                        opts.GeneratedCodeMode = TypeLoadMode.Auto;
+                        opts.GeneratedCodeOutputPath = tempDir;
+                    });
+                })
+                .StartAsync();
+
+            // Resolve the secondary stores — this triggers Build() which calls
+            // InitializeSynchronously() and writes generated code to
+            // {tempDir}/{StoreName}/Stores/ when SourceCodeWritingEnabled is true
+            host.Services.GetRequiredService<IBug4185Store>();
+            host.Services.GetRequiredService<IBug4185OtherStore>();
+
+            // Now simulate "dotnet run -- codegen write" using DynamicCodeBuilder.
+            // This writes generated code via each ICodeFileCollection's Rules,
+            // including SecondaryDocumentStores which writes to {tempDir}/Stores/
+            var collections = host.Services.GetServices<ICodeFileCollection>().ToArray();
+            var codeBuilder = new DynamicCodeBuilder(host.Services, collections);
+            codeBuilder.WriteGeneratedCode(_ => { });
+
+            // Collect all generated .cs files
+            var allFiles = Directory.GetFiles(tempDir, "*.cs", SearchOption.AllDirectories);
+
+            // Check that each generated file's fully qualified type name is unique.
+            // Read each file, extract the namespace + class name, and detect conflicts
+            // where the same fully-qualified type is generated to multiple locations.
+            var typeLocations = allFiles
+                .Select(f => new
+                {
+                    Path = f.Replace(tempDir, ""),
+                    Content = File.ReadAllText(f)
+                })
+                .Select(f =>
+                {
+                    var nsMatch = System.Text.RegularExpressions.Regex.Match(f.Content, @"namespace\s+([\w.]+)");
+                    var classMatch = System.Text.RegularExpressions.Regex.Match(f.Content, @"class\s+(\w+)");
+                    return new
+                    {
+                        f.Path,
+                        FullyQualifiedName = nsMatch.Success && classMatch.Success
+                            ? $"{nsMatch.Groups[1].Value}.{classMatch.Groups[1].Value}"
+                            : null
+                    };
+                })
+                .Where(f => f.FullyQualifiedName != null)
+                .ToList();
+
+            var duplicates = typeLocations
+                .GroupBy(f => f.FullyQualifiedName)
+                .Where(g => g.Count() > 1)
+                .ToList();
+
+            duplicates.ShouldBeEmpty(
+                "codegen write produced duplicate types at different locations:\n" +
+                string.Join("\n", duplicates.Select(g =>
+                    $"  {g.Key}:\n" +
+                    string.Join("\n", g.Select(f => "    " + f.Path)))));
+        }
+        finally
+        {
+            if (Directory.Exists(tempDir))
+            {
+                Directory.Delete(tempDir, true);
+            }
+        }
+    }
+
+    [Fact]
+    public async Task projection_with_secondary_store_dependency_should_work_at_runtime()
+    {
+        using var host = await Host.CreateDefaultBuilder()
+            .ConfigureServices(services =>
+            {
+                services.AddMartenStore<IBug4185Store>(opts =>
+                {
+                    opts.Connection(ConnectionSource.ConnectionString);
+                    opts.DatabaseSchemaName = "bug4185_sec";
+                    opts.GeneratedCodeMode = TypeLoadMode.Auto;
+                });
+
+                services.AddMarten(opts =>
+                    {
+                        opts.Connection(ConnectionSource.ConnectionString);
+                        opts.DatabaseSchemaName = "bug4185_pri";
+                        opts.GeneratedCodeMode = TypeLoadMode.Auto;
+                    })
+                    .AddProjectionWithServices<OrderProjection4185>(
+                        ProjectionLifecycle.Inline,
+                        ServiceLifetime.Singleton)
+                    .ApplyAllDatabaseChangesOnStartup();
+            })
+            .StartAsync();
+
+        var store = host.Services.GetRequiredService<IDocumentStore>();
+        var streamId = Guid.NewGuid();
+        await using (var session = store.LightweightSession())
+        {
+            session.Events.StartStream(streamId, new OrderPlaced4185("Widget", 9.99m, 3));
+            await session.SaveChangesAsync();
+        }
+
+        await using (var session = store.QuerySession())
+        {
+            var summary = await session.LoadAsync<OrderSummary4185>(streamId);
+            summary.ShouldNotBeNull();
+            summary.ProductName.ShouldBe("Widget");
+            summary.Total.ShouldBe(29.97m);
+        }
+    }
+}
diff --git a/src/Marten/Internal/SecondaryStoreConfig.cs b/src/Marten/Internal/SecondaryStoreConfig.cs
index dbeb3af038..b78f1bbe5b 100644
--- a/src/Marten/Internal/SecondaryStoreConfig.cs
+++ b/src/Marten/Internal/SecondaryStoreConfig.cs
@@ -121,6 +121,11 @@ public T Build(IServiceProvider provider)
         var rules = options.CreateGenerationRules();
 
         rules.GeneratedNamespace = SchemaConstants.MartenGeneratedNamespace;
+        // CreateGenerationRules() appends StoreName to the output path, but
+        // SecondaryDocumentStores.Rules (used by codegen write) strips it via
+        // ParentDirectory(). Align the paths to avoid writing duplicate files
+        // with the same namespace and class name to different directories (#4185)
+        rules.GeneratedCodeOutputPath = rules.GeneratedCodeOutputPath.ParentDirectory();
         this.InitializeSynchronously(rules, Parent, provider);
 
         var store = (T)Activator.CreateInstance(_storeType!, options)!;