Write each plane via once in the exact engine's output too

The exact engine rebuilds its output from each connection's own vias,
so connections sharing a plane via each wrote a copy: th-eda's flight
controller had 71 spots carrying 2 to 4 vias of one net. Vias are now
written once per net and spot, and never on a via already on the board.

tests/route/test_via_spacing.py

@@ -34,3 +34,23 @@ def test_a_nets_own_vias_keep_hole_to_hole_spacing():
     need = (300 + 250) * scale  # drill + 0.25 mm, centre to centre
     for a, b in itertools.combinations(vias, 2):
         assert math.hypot(a.x - b.x, a.y - b.y) >= need, f"vias at {a} and {b} too close"
+
+
+def test_connections_sharing_a_plane_via_write_it_once():
+    # eight GND pads 0.4 mm apart: the hole spacing makes several of them
+    # end on the same plane via, which must come out once, not once each
+    pads = " ".join(f"(place G{i} {9000 + 400 * i} -10000 front 0)" for i in range(8))
+    pins = " ".join(f"G{i}-1" for i in range(8))
+    dsn = parse_dsn(
+        _DSN.replace(
+            "(place G1 9000 -10000 front 0) (place G2 9500 -10000 front 0)\n"
+            "                          (place G3 10000 -10000 front 0) (place G4 10500 -10000 front 0)",
+            pads,
+        )
+        .replace("(pins G1-1 G2-1 G3-1 G4-1)", f"(pins {pins})")
+        .replace("(rect F.Cu -150 -300 150 300)", "(rect F.Cu -125 -300 125 300)")
+    )
+    exact, _scale, _ = route_dsn_board_exact(dsn, resolution=2)
+    vias = exact.result.vias[1]
+    assert len(vias) < 8, "no via is shared, so this board no longer tests sharing"
+    assert len(set(vias)) == len(vias), "a shared via written more than once"