Drop duplicate HV buses for eHv clustering busmap

When scn_extension is selected, some HV buses are connected to more than one eHV bus.
This leads to duplicated indices in the path matrix. To avoid this, each HV bus is
only considered once using transformers.bus1.unique().

etrago/cluster/spatial.py

@@ -429,7 +429,7 @@ def busmap_by_shortest_path(network, fromlvl, tolvl, cpu_cores=4):
     lines_plus_dc["carrier"] = "AC"
 
     # temporary end points, later replaced by bus1 pendant
-    t_buses = transformer[mask].bus0
+    t_buses = transformer[mask].bus0.unique()
 
     # create all possible pathways
     ppaths = list(product(s_buses, t_buses))