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.

src/freeroute/route/exact_router.py

@@ -31,7 +31,7 @@ from dataclasses import dataclass, field
 import math
 
 from freeroute.board.board import BasicBoard
-from freeroute.board.items import Pin
+from freeroute.board.items import Pin, Via
 from freeroute.board.search_tree import ShapeSearchTree, TreeShape
 from freeroute.geometry import (
     IntBox,
@@ -178,6 +178,11 @@ def route_board_exact(
 
     via_layers = frozenset(signal_layers)
     accepted: dict[int, _Accepted] = {}
+    fixed_vias = {
+        (item.net_nos[0], item.location)
+        for item in board.get_items()
+        if isinstance(item, Via) and item.fixed and item.net_nos
+    }
 
     # Verify connection by connection, so one conflicting segment of a
     # 25-pin supply net costs that one connection, not the whole net.
@@ -207,7 +212,11 @@ def route_board_exact(
         if acc is None:
             acc = accepted[net_no] = _Accepted(wires=[], vias=[])
         acc.wires.extend(wires)
-        acc.vias.extend(vias)
+        # connections of one net that end on the same plane cell share its
+        # via: write it once, and never on top of a via already on the board
+        acc.vias.extend(
+            v for v in dict.fromkeys(vias) if v not in acc.vias and (net_no, v) not in fixed_vias
+        )
         for box, box_layers in boxes:
             oid = next(owner)
             tree.insert(TreeShape(oid, net_no, box_layers, box, routed=True))