[TS] Prevent generics to be duplicated (fable-compiler#3942)

src/Fable.Cli/CHANGELOG.md

@@ -24,6 +24,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * [JS/TS/Python/Rust] Fixed String.StartsWith/EndsWith (#3934) (by @ncave)
 * [All/Rust] Removed Regex.Replace from hot paths (by @ncave)
 * [JS] Fix regression, generate `let` variable when using `import` on a private mutable variable (by @MangelMaxime)
+* [TS] Prevent generics to be duplicated (by @MangelMaxime)
 
 ## 4.22.0 - 2024-10-02
 

src/Fable.Transforms/Fable2Babel.fs

@@ -565,6 +565,7 @@ module Annotation =
                 TypeParameter.typeParameter (name, ?bound = bound) |> Some
             | _ -> None
         )
+        |> Array.distinct
 
     let makeTypeParamInstantiation (com: IBabelCompiler) ctx genArgs =
         if List.isEmpty genArgs then

tests/Js/Main/DictionaryTests.fs

@@ -12,6 +12,17 @@ type MyRecord = { a: int }
 
 type R = { i: int; s: string }
 
+type Table<'K,'V when 'K:equality> () =
+
+    let dic = new Dictionary<'K,'V>()
+
+    let addKeyValue key value =
+        dic.[key] <- value
+
+    member _.add key value =
+        addKeyValue key value
+
+    member _.Dic = dic
 let tests =
   testList "Dictionaries" [
     testCase "Dictionary KeyValuePattern works" <| fun () -> // See #509
@@ -251,4 +262,11 @@ let tests =
         equal cache.[typeof<int>] 1
         equal cache.[typeof<string>] 2
         equal cache.[typeof<int64>] 3
+
+    testCase "Check that generics are not duplicated " <| fun _ -> // See #3911
+        let table = Table<_,_>()
+        table.add "A" 1
+        table.add "B" 2
+        table.add "C" 3
+        table.Dic.Count |> equal 3
   ]