diff --git a/docs/release-notes/.FSharp.Compiler.Service/11.0.100.md b/docs/release-notes/.FSharp.Compiler.Service/11.0.100.md
index 965957dbc0c..313a0cf7deb 100644
--- a/docs/release-notes/.FSharp.Compiler.Service/11.0.100.md
+++ b/docs/release-notes/.FSharp.Compiler.Service/11.0.100.md
@@ -1,5 +1,6 @@
 ### Fixed
 
+* Report `FS0037 Duplicate definition of type or module` at type-check time when two sibling modules in a `module rec` / `namespace rec` group share a name, instead of letting the duplicate slip through to IL emit where it surfaced as the cryptic `FS2014: duplicate entry . in type index table`. ([Issue #6694](https://github.com/dotnet/fsharp/issues/6694), [PR #19913](https://github.com/dotnet/fsharp/pull/19913))
 * Suppress hover/symbol resolution for wildcard `_` patterns inside `member _.…` bodies that incorrectly showed `val _: T` tooltip. ([PR #19760](https://github.com/dotnet/fsharp/pull/19760))
 * Deduplicate format specifier locations in computation expressions so editor tooling no longer reports duplicate entries for the same `%` specifier. ([Issue #16419](https://github.com/dotnet/fsharp/issues/16419), [PR #19791](https://github.com/dotnet/fsharp/pull/19791))
 * Reject non-function bindings for single-case and partial active pattern names with FS1209, matching the existing multi-case behavior. ([PR #19763](https://github.com/dotnet/fsharp/pull/19763))
diff --git a/src/Compiler/Checking/CheckDeclarations.fs b/src/Compiler/Checking/CheckDeclarations.fs
index 4a4361e9808..1811bda062d 100644
--- a/src/Compiler/Checking/CheckDeclarations.fs
+++ b/src/Compiler/Checking/CheckDeclarations.fs
@@ -4095,6 +4095,11 @@ module EstablishTypeDefinitionCores =
         let envTmp, withEnvs =  
             (envInitial, withEntities) ||> MutRecShapes.computeEnvs 
               (fun envAbove (MutRecDefnsPhase2DataForModule (moduleTyAcc, moduleEntity)) ->  
+                  // In recursive scopes all siblings build their entities in Phase1A against the same
+                  // envInitial, so duplicate sibling-module names are not yet visible there. Detect
+                  // them here at the publish point, where envAbove's accumulator already contains
+                  // any earlier-published siblings. See https://github.com/dotnet/fsharp/issues/6694.
+                  CheckForDuplicateModule envAbove moduleEntity.DemangledModuleOrNamespaceName moduleEntity.Range
                   PublishModuleDefn cenv envAbove moduleEntity 
                   MakeInnerEnvWithAcc true envAbove moduleEntity.Id moduleTyAcc moduleEntity.ModuleOrNamespaceType.ModuleOrNamespaceKind)
               (fun envAbove _ -> envAbove)
diff --git a/tests/FSharp.Compiler.ComponentTests/ErrorMessages/ModuleTests.fs b/tests/FSharp.Compiler.ComponentTests/ErrorMessages/ModuleTests.fs
index 4ebb36eac5c..e5c76ccd542 100644
--- a/tests/FSharp.Compiler.ComponentTests/ErrorMessages/ModuleTests.fs
+++ b/tests/FSharp.Compiler.ComponentTests/ErrorMessages/ModuleTests.fs
@@ -191,4 +191,100 @@ AutoOpen>] L1 = do ()
         |> shouldFail
         |> withDiagnostics [
             Error 10, Line 3, Col 1, Line 3, Col 9, "Unexpected start of structured construct in attribute list"
-        ]
\ No newline at end of file
+        ]
+
+    [<Fact>]
+    let ``Recursive module with duplicate sibling modules emits FS0037`` () =
+        FSharp """
+module rec A
+
+module Foobar = let x = 1
+module Foobar = let y = 2
+"""
+        |> typecheck
+        |> shouldFail
+        |> withErrorCode 37
+        |> withDiagnosticMessageMatches "Foobar"
+        |> ignore
+
+    [<Fact>]
+    let ``Recursive namespace with duplicate sibling modules emits FS0037`` () =
+        FSharp """
+namespace rec N
+
+module Foobar = let x = 1
+module Foobar = let y = 2
+"""
+        |> typecheck
+        |> shouldFail
+        |> withErrorCode 37
+        |> withDiagnosticMessageMatches "Foobar"
+        |> ignore
+
+    [<Fact>]
+    let ``Recursive module nested cousins with same name compile`` () =
+        FSharp """
+module rec Outer
+
+module A =
+    module Inner = let x = 1
+
+module B =
+    module Inner = let y = 2
+"""
+        |> typecheck
+        |> shouldSucceed
+
+    [<Fact>]
+    let ``Recursive module nested siblings with duplicate name emit FS0037`` () =
+        FSharp """
+module rec Outer
+
+module Inner =
+    module Dup = let x = 1
+    module Dup = let y = 2
+"""
+        |> typecheck
+        |> shouldFail
+        |> withErrorCode 37
+        |> withDiagnosticMessageMatches "Dup"
+        |> ignore
+
+    [<Fact>]
+    let ``Non-recursive module with duplicate sibling modules still emits FS0037`` () =
+        FSharp """
+module A
+
+module Foobar = let x = 1
+module Foobar = let y = 2
+"""
+        |> typecheck
+        |> shouldFail
+        |> withErrorCode 37
+        |> withDiagnosticMessageMatches "Foobar"
+        |> ignore
+
+    [<Fact>]
+    let ``Single file with two namespace fragments and different modules compiles`` () =
+        FSharp """
+namespace N
+
+module M = let x = 1
+
+namespace N
+
+module M2 = let y = 2
+"""
+        |> typecheck
+        |> shouldSucceed
+
+    [<Fact>]
+    let ``Recursive module siblings differing only in case compile`` () =
+        FSharp """
+module rec A
+
+module Foobar = let x = 1
+module FooBar = let y = 2
+"""
+        |> typecheck
+        |> shouldSucceed
\ No newline at end of file