diff --git a/test/test_transform.py b/test/test_transform.py
index 8060d2038..4ad1971fb 100644
--- a/test/test_transform.py
+++ b/test/test_transform.py
@@ -1704,6 +1704,35 @@ def test_precompute_lets_inner_length1_inames_live():
             == parse("(e_inner + e_outer*16) / i_0"))
 
 
+def test_duplicate_iname_not_read_only_nested(ctx_factory):
+    # See <https://github.com/inducer/loopy/issues/859>
+    ctx = ctx_factory()
+    t_unit = lp.make_kernel(
+        "{[i, j]: 0<=i,j<10}",
+        """
+        for i
+            <> acc = 0 {id=init, tags=foo}
+            for j
+                acc = acc + A[i, j] * x[i, j] {id=update, tags=foo}
+            end
+            y[i] = acc {id=assign, tags=foo}
+        end
+        """,
+        [lp.GlobalArg("A,x,y", shape=lp.auto, dtype=np.float32),
+         ...],
+    )
+    ref_t_unit = t_unit
+
+    t_unit = lp.duplicate_inames(
+        t_unit,
+        inames="i", within="tag:foo", new_inames="irow")
+    print(t_unit)
+    assert (t_unit.default_entrypoint.id_to_insn["init"].within_inames
+            == frozenset({"irow"}))
+
+    lp.auto_test_vs_ref(ref_t_unit, ctx, t_unit)
+
+
 if __name__ == "__main__":
     if len(sys.argv) > 1:
         exec(sys.argv[1])