diff --git a/CHANGES.md b/CHANGES.md
index 8bfca878e..5768750b3 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -18,6 +18,9 @@
 
 ### Fixed
 
+- Error in case where multiple packages with different dev-repo fields would be
+  placed in the same duniverse directory (#377, @gridbugs)
+
 ### Removed
 
 ### Security
diff --git a/lib/dev_repo.ml b/lib/dev_repo.ml
index 2781ce180..30fbea196 100644
--- a/lib/dev_repo.ml
+++ b/lib/dev_repo.ml
@@ -5,6 +5,7 @@ type t = string
 let compare = String.compare
 let from_string s = s
 let to_string t = t
+let pp = Fmt.string
 
 let rec repeat_while_some x ~f =
   match f x with None -> x | Some x -> repeat_while_some x ~f
diff --git a/lib/dev_repo.mli b/lib/dev_repo.mli
index cc1a59f3f..e74bb1cc4 100644
--- a/lib/dev_repo.mli
+++ b/lib/dev_repo.mli
@@ -4,6 +4,7 @@ type t
 
 val from_string : string -> t
 val to_string : t -> string
+val pp : t Fmt.t
 
 val repo_name : t -> (string, Rresult.R.msg) result
 (** Computes a name for the repo by applying the following method:
diff --git a/lib/duniverse.ml b/lib/duniverse.ml
index 5986a12dc..5abac2f9a 100644
--- a/lib/duniverse.ml
+++ b/lib/duniverse.ml
@@ -231,6 +231,54 @@ let dev_repo_map_from_packages packages =
         | Some pkgs -> Some (pkg :: pkgs)
         | None -> Some [ pkg ]))
 
+(* converts a map from dev-repos to lists of packages to a list of repos after
+   checking for errors *)
+let dev_repo_package_map_to_repos dev_repo_package_map =
+  let open Result.O in
+  let dev_repo_to_repo_result_map =
+    Dev_repo.Map.mapi dev_repo_package_map ~f:(fun dev_repo pkgs ->
+        Repo.from_packages ~dev_repo pkgs)
+  in
+  (* Handle any errors relating to individual repos but maintain the
+     association between dev-repo and repo for further error checking. *)
+  let* repo_by_dev_repo =
+    Dev_repo.Map.bindings dev_repo_to_repo_result_map
+    |> List.map ~f:(fun (dev_repo, repo_result) ->
+           Result.map (fun repo -> (dev_repo, repo)) repo_result)
+    |> Base.Result.all
+  in
+  (* Detect the case where multiple different dev-repos are associated with the
+     same duniverse directory. *)
+  let* () =
+    let dev_repos_by_dir =
+      List.fold_left repo_by_dev_repo ~init:String.Map.empty
+        ~f:(fun acc (dev_repo, (repo : _ Repo.t)) ->
+          String.Map.update acc repo.dir ~f:(function
+            | None -> Some [ dev_repo ]
+            | Some dev_repos -> Some (dev_repo :: dev_repos)))
+      |> String.Map.bindings
+    in
+    match
+      List.find_opt dev_repos_by_dir ~f:(fun (_, dev_repos) ->
+          List.length dev_repos > 1)
+    with
+    | None -> Ok ()
+    | Some (dir, dev_repos) ->
+        let dir_path = Fpath.(Config.vendor_dir / dir) in
+        let message_first_line =
+          Fmt.str "Multiple dev-repos would be vendored into the directory: %a"
+            Fpath.pp dir_path
+        in
+        let dev_repos_pp =
+          Fmt.list
+            ~sep:Fmt.(const char '\n')
+            (fun ppf dev_repo -> Fmt.pf ppf "- %a" Dev_repo.pp dev_repo)
+        in
+        Rresult.R.error_msgf "%s\nDev-repos:\n%a" message_first_line
+          dev_repos_pp dev_repos
+  in
+  Ok (List.map ~f:snd repo_by_dev_repo)
+
 let from_dependency_entries ~get_default_branch dependencies =
   let open Result.O in
   let summaries =
@@ -247,10 +295,7 @@ let from_dependency_entries ~get_default_branch dependencies =
   let* pkg_opts = Base.Result.all results in
   let pkgs = Base.List.filter_opt pkg_opts in
   let dev_repo_map = dev_repo_map_from_packages pkgs in
-  Dev_repo.Map.fold dev_repo_map ~init:[]
-    ~f:(fun ~key:dev_repo ~data:pkgs acc ->
-      Repo.from_packages ~dev_repo pkgs :: acc)
-  |> Base.Result.all
+  dev_repo_package_map_to_repos dev_repo_map
 
 let resolve ~resolve_ref t =
   Parallel.map ~f:(Repo.resolve ~resolve_ref) t |> Base.Result.all
diff --git a/test/bin/error-on-directory-conflict.t/repo/packages/bar/bar.0.1.0/opam b/test/bin/error-on-directory-conflict.t/repo/packages/bar/bar.0.1.0/opam
new file mode 100644
index 000000000..9ebd9593a
--- /dev/null
+++ b/test/bin/error-on-directory-conflict.t/repo/packages/bar/bar.0.1.0/opam
@@ -0,0 +1,12 @@
+opam-version: "2.0"
+dev-repo: "git+https://github.com/bar/project"
+depends: [
+  "dune"
+]
+build: [ "dune" "build" ]
+url {
+  src: "https://test.com/bar.tbz"
+  checksum: [
+    "sha256=0000000000000000000000000000000000000000000000000000000000000000"
+  ]
+}
diff --git a/test/bin/error-on-directory-conflict.t/repo/packages/foo-branch/foo-branch.0.1.0/opam b/test/bin/error-on-directory-conflict.t/repo/packages/foo-branch/foo-branch.0.1.0/opam
new file mode 100644
index 000000000..c410ea5ff
--- /dev/null
+++ b/test/bin/error-on-directory-conflict.t/repo/packages/foo-branch/foo-branch.0.1.0/opam
@@ -0,0 +1,12 @@
+opam-version: "2.0"
+dev-repo: "git+https://github.com/foo/project.git#branch"
+depends: [
+  "dune"
+]
+build: [ "dune" "build" ]
+url {
+  src: "https://test.com/foo-branch.tbz"
+  checksum: [
+    "sha256=0000000000000000000000000000000000000000000000000000000000000000"
+  ]
+}
diff --git a/test/bin/error-on-directory-conflict.t/repo/packages/foo/foo.0.1.0/opam b/test/bin/error-on-directory-conflict.t/repo/packages/foo/foo.0.1.0/opam
new file mode 100644
index 000000000..fba9fcf69
--- /dev/null
+++ b/test/bin/error-on-directory-conflict.t/repo/packages/foo/foo.0.1.0/opam
@@ -0,0 +1,12 @@
+opam-version: "2.0"
+dev-repo: "git+https://github.com/foo/project"
+depends: [
+  "dune"
+]
+build: [ "dune" "build" ]
+url {
+  src: "https://test.com/foo.tbz"
+  checksum: [
+    "sha256=0000000000000000000000000000000000000000000000000000000000000000"
+  ]
+}
diff --git a/test/bin/error-on-directory-conflict.t/run.t b/test/bin/error-on-directory-conflict.t/run.t
new file mode 100644
index 000000000..bcaeef795
--- /dev/null
+++ b/test/bin/error-on-directory-conflict.t/run.t
@@ -0,0 +1,16 @@
+We have a project which depends on multiple packages with different dev-repos
+but which happen to resolve to the same directory under duniverse. This tests
+that such a situation results in an explicit error.
+
+  $ gen-minimal-repo
+  $ opam-monorepo lock
+  ==> Using 1 locally scanned package as the target.
+  ==> Found 11 opam dependencies for the target package.
+  ==> Querying opam database for their metadata and Dune compatibility.
+  ==> Calculating exact pins for each of them.
+  opam-monorepo: [ERROR] Multiple dev-repos would be vendored into the directory: duniverse/project
+  Dev-repos:
+  - git+https://github.com/foo/project.git#branch
+  - git+https://github.com/foo/project
+  - git+https://github.com/bar/project
+  [1]
diff --git a/test/bin/error-on-directory-conflict.t/x.opam b/test/bin/error-on-directory-conflict.t/x.opam
new file mode 100644
index 000000000..d9a0fa45b
--- /dev/null
+++ b/test/bin/error-on-directory-conflict.t/x.opam
@@ -0,0 +1,10 @@
+opam-version: "2.0"
+depends: [
+  "foo"
+  "foo-branch"
+  "bar"
+]
+x-opam-monorepo-opam-repositories: [
+  "file://$OPAM_MONOREPO_CWD/minimal-repo"
+  "file://$OPAM_MONOREPO_CWD/repo"
+]