diff --git a/src/compiler/ir.ml b/src/compiler/ir.ml
index 1c04e80..03f6c7a 100644
--- a/src/compiler/ir.ml
+++ b/src/compiler/ir.ml
@@ -78,12 +78,21 @@ let rep t n m =
       | Ok t -> Ok (Rep (t, n, m)))
 
 let seq a b =
-  match (a, b) with
-    | Eps, x | x, Eps -> x
-    | Seq l1, Seq l2 -> Seq (l1 @ l2)
-    | Seq l1, x -> Seq (l1 @ [x])
-    | x, Seq l2 -> Seq (x :: l2)
-    | _ -> Seq [a; b]
+  let an = capture_names a in
+  let bn = capture_names b in
+  let dups = SSet.inter an bn in
+  if not (SSet.is_empty dups) then
+    Error
+      (Printf.sprintf "'as' binding '%s' is bound in multiple positions"
+         (SSet.choose dups))
+  else
+    Ok
+      (match (a, b) with
+        | Eps, x | x, Eps -> x
+        | Seq l1, Seq l2 -> Seq (l1 @ l2)
+        | Seq l1, x -> Seq (l1 @ [x])
+        | x, Seq l2 -> Seq (x :: l2)
+        | _ -> Seq [a; b])
 
 let alt a b =
   let an = capture_names a in
@@ -110,7 +119,12 @@ let check_invariant t =
         SSet.add name names
     | Seq elems ->
         assert (List.length elems >= 2);
-        List.fold_left (fun acc x -> SSet.union acc (check x)) SSet.empty elems
+        List.fold_left
+          (fun acc x ->
+            let names = check x in
+            assert (SSet.is_empty (SSet.inter acc names));
+            SSet.union acc names)
+          SSet.empty elems
     | Alt [] | Alt [_] -> assert false
     | Alt (first :: rest) ->
         let names = check first in
diff --git a/src/compiler/ir.mli b/src/compiler/ir.mli
index 4a498b4..2722874 100644
--- a/src/compiler/ir.mli
+++ b/src/compiler/ir.mli
@@ -35,10 +35,11 @@ type t =
     Constructors that enforce structural invariants return [result].
     - {!alt} checks name consistency across branches.
     - {!capture} checks for shadowed inner bindings.
+    - {!seq} rejects duplicate capture names across elements.
     - {!star}, {!plus}, {!rep} reject inner captures. *)
 
 val chars : Cset.t -> t
-val seq : t -> t -> t
+val seq : t -> t -> (t, string) result
 val alt : t -> t -> (t, string) result
 val star : t -> (t, string) result
 val plus : t -> (t, string) result
diff --git a/src/syntax/ppx_sedlex.ml b/src/syntax/ppx_sedlex.ml
index 33df3bc..5e19306 100644
--- a/src/syntax/ppx_sedlex.ml
+++ b/src/syntax/ppx_sedlex.ml
@@ -562,7 +562,7 @@ let ir_of_pattern env =
       (* (p1, p2, ...) — sequence *)
       | Ppat_tuple (p :: pl) ->
           List.fold_left
-            (fun acc p -> Ir.seq acc (aux ~encoding p))
+            (fun acc p -> unwrap p.ppat_loc (Ir.seq acc (aux ~encoding p)))
             (aux ~encoding p) pl
       (* Star p — zero-or-more repetition *)
       | Ppat_construct ({ txt = Lident "Star"; _ }, Some (_, p)) ->
@@ -682,7 +682,9 @@ let ir_of_pattern env =
             | Pconst_string (s, _, _) ->
                 let rev_l = rev_csets_of_string s ~loc:p.ppat_loc ~encoding in
                 List.fold_left
-                  (fun acc cset -> Ir.seq (Ir.chars cset) acc)
+                  (fun acc cset ->
+                    (* chars have no captures, so seq cannot fail *)
+                    Result.get_ok (Ir.seq (Ir.chars cset) acc))
                   Ir.eps rev_l
             | Pconst_char c -> Ir.chars (char c)
             | Pconst_integer (i, _) ->
diff --git a/test/codegen/test_errors.ml b/test/codegen/test_errors.ml
index 1481898..f508a49 100644
--- a/test/codegen/test_errors.ml
+++ b/test/codegen/test_errors.ml
@@ -89,6 +89,17 @@ let%expect_test "error: as shadows inner binding" =
     Error: Sedlex: 'as' binding 'x' shadows an inner binding of the same name
     |}]
 
+let%expect_test "error: duplicate as binding in sequence" =
+  [%compile_error
+    [%sedlex match buf with ('a' as x), 'b', ('c' as x) -> ignore x | _ -> ()]];
+  [%expect
+    {|
+    File "test/codegen/test_errors.ml", characters 45-55:
+       |     [%sedlex match buf with ('a' as x), 'b', ('c' as x) -> ignore x | _ -> ()]];
+                                                      ^^^^^^^^^^
+    Error: Sedlex: 'as' binding 'x' is bound in multiple positions
+    |}]
+
 let%expect_test "error: different names in or-pattern" =
   [%compile_error
     [%sedlex
@@ -96,8 +107,8 @@ let%expect_test "error: different names in or-pattern" =
   [%expect
     {|
     File "test/codegen/test_errors.ml", characters 21-44:
-       |       match buf with ('a' as x) | ('b' as y) -> ignore (x, y) | _ -> ()]];
-                              ^^^^^^^^^^^^^^^^^^^^^^^
+        |       match buf with ('a' as x) | ('b' as y) -> ignore (x, y) | _ -> ()]];
+                               ^^^^^^^^^^^^^^^^^^^^^^^
     Error: Sedlex: all branches of '|' must bind the same names with 'as'
     |}]