diff --git a/src/dune_project.ml b/src/dune_project.ml
index 4be06c51c4ed..e185ee63580b 100644
--- a/src/dune_project.ml
+++ b/src/dune_project.ml
@@ -576,7 +576,7 @@ let parse ~dir ~lang ~packages ~file =
      and+ source = field_o "source" (Syntax.since Stanza.syntax (1, 7)
                                      >>> Source_kind.decode)
      and+ dune_defined_packages =
-       multi_field "package" (package_decode ~dir)
+       multi_field "package" (located (package_decode ~dir))
      and+ authors = field ~default:[] "authors"
                       (Syntax.since Stanza.syntax (1, 9) >>> repeat string)
      and+ license = field_o "license"
@@ -614,8 +614,15 @@ let parse ~dir ~lang ~packages ~file =
      let allow_approx_merlin =
        Option.value ~default:false allow_approx_merlin in
      let packages =
-       List.fold_left ~init:packages ~f:(fun acc (p : Package.t) ->
-         Package.Name.Map.add acc p.name p) dune_defined_packages
+       match
+        dune_defined_packages
+        |> Package.Name.Map.of_list_map
+             ~f:(fun (_loc, (p : Package.t)) -> p.name, p)
+       with
+       | Error (p, (_, _), (loc, _)) ->
+         of_sexp_errorf loc "package %s is already defined"
+           (Package.Name.to_string p)
+       | Ok d -> Package.Name.Map.superpose packages d
      in
      { name
      ; root = dir