diff --git a/src/install_rules.ml b/src/install_rules.ml
index 84797e56..247bbb5f 100644
--- a/src/install_rules.ml
+++ b/src/install_rules.ml
@@ -45,9 +45,9 @@ module Gen(P : Install_params) = struct
 
   let init_meta () =
     let public_libs = Lib.DB.all (SC.public_libs sctx) in
-    List.iter public_libs ~f:gen_lib_dune_file;
-    List.map public_libs ~f:(fun lib ->
-      (Findlib.root_package_name (Lib.name lib), lib))
+    Lib.Set.iter public_libs ~f:gen_lib_dune_file;
+    Lib.Set.to_list public_libs
+    |> List.map ~f:(fun lib -> (Findlib.root_package_name (Lib.name lib), lib))
     |> String_map.of_list_multi
     |> String_map.merge (SC.packages sctx) ~f:(fun _name pkg libs ->
       let pkg  = Option.value_exn pkg          in
diff --git a/src/lib.ml b/src/lib.ml
index 3e04aedc..de81d79f 100644
--- a/src/lib.ml
+++ b/src/lib.ml
@@ -319,6 +319,12 @@ let to_id t : Id.t =
   ; name      = t.name
   }
 
+module Set = Set.Make(
+struct
+  type nonrec t = t
+  let compare x y = compare x.unique_id y.unique_id
+end)
+
 module L = struct
   type nonrec t = t list
 
@@ -973,13 +979,13 @@ module DB = struct
 
   let rec all ?(recursive=false) t =
     let l =
-      List.filter_map (Lazy.force t.all) ~f:(fun name ->
+      List.fold_left (Lazy.force t.all) ~f:(fun libs name ->
         match find t name with
-        | Ok    x -> Some x
-        | Error _ -> None)
+        | Ok    x -> Set.add libs x
+        | Error _ -> libs) ~init:Set.empty
     in
     match recursive, t.parent with
-    | true, Some t -> all ~recursive t @ l
+    | true, Some t -> Set.union (all ~recursive t) l
     | _ -> l
 end
 
diff --git a/src/lib.mli b/src/lib.mli
index 0cb112ea..14ad81ac 100644
--- a/src/lib.mli
+++ b/src/lib.mli
@@ -30,6 +30,8 @@ val jsoo_runtime : t -> Path.t list
     the process *)
 val unique_id : t -> int
 
+module Set : Set.S with type elt = t
+
 module Status : sig
   type t =
     | Installed
@@ -265,7 +267,7 @@ module DB : sig
   (** Return the list of all libraries in this database. If
       [recursive] is true, also include libraries in parent databases
       recursively. *)
-  val all : ?recursive:bool -> t -> lib list
+  val all : ?recursive:bool -> t -> Set.t
 end with type lib := t
 
 (** {1 Transitive closure} *)