diff --git a/tests/route/test_fixed_wiring.py b/tests/route/test_fixed_wiring.py
index aa1bee8..d5bdfbe 100644
--- a/tests/route/test_fixed_wiring.py
+++ b/tests/route/test_fixed_wiring.py
@@ -91,3 +91,18 @@ def test_a_fanned_out_signal_is_routed_from_its_via():
     assert sig in exact.result.routed_net_numbers
     assert not exact.result.vias.get(sig), "the fan-out via is already there"
     assert {names[layer] for layer, _ in exact.result.wires[sig]} == {"B.Cu"}
+
+
+def test_the_session_never_repeats_copper_already_on_the_board():
+    # the JAR writes the DSN's wiring back into its session, and an applier
+    # then adds it a second time on top of itself; freeroute writes only
+    # what it routed
+    from freeroute.route.pipeline import build_exact_routing_result
+
+    fixed_vias = {(5000.0, -11500.0), (10000.0, -11500.0)}
+    fixed_paths = {(5000.0, -10000.0, 5000.0, -11500.0), (10000.0, -10000.0, 10000.0, -11500.0),
+                   (2000.0, -2000.0, 8000.0, -2000.0)}
+    result = build_exact_routing_result(parse_dsn(_FANOUT), resolution=2)
+    for net in result.nets:
+        assert not {(v.x, v.y) for v in net.vias} & fixed_vias
+        assert not {tuple(w.coords) for w in net.wires} & fixed_paths