print warning to console if redundant switching station is found

and where it is

only prints the warning once per redundant station, per minetest
session. if there are multiple redundant stations on a network, digging
one should cause the warning to appear for the next one technic finds,
if it doesn't show them all right away.

technic/machines/switching_station.lua

@@ -2,6 +2,7 @@
 
 technic.networks = {}
 technic.cables = {}
+technic.redundant_warn = {}
 
 local mesecons_path = minetest.get_modpath("mesecons")
 local digilines_path = minetest.get_modpath("digilines")
@@ -44,11 +45,15 @@ minetest.register_node("technic:switching_station",{
 		meta:set_string("active", 1)
 		meta:set_string("channel", "switching_station"..minetest.pos_to_string(pos))
 		meta:set_string("formspec", "field[channel;Channel;${channel}]")
+		local poshash = minetest.hash_node_position(pos)
+		technic.redundant_warn.poshash = nil
 	end,
 	after_dig_node = function(pos)
 		minetest.forceload_free_block(pos)
 		pos.y = pos.y - 1
 		minetest.forceload_free_block(pos)
+		local poshash = minetest.hash_node_position(pos)
+		technic.redundant_warn.poshash = nil
 	end,
 	on_receive_fields = function(pos, formname, fields, sender)
 		if not fields.channel then
@@ -234,6 +239,13 @@ minetest.register_abm({
 			minetest.forceload_free_block(pos)
 			minetest.forceload_free_block(pos1)
 			meta:set_string("infotext",S("%s Already Present"):format(machine_name))
+
+			local poshash = minetest.hash_node_position(pos)
+
+			if not technic.redundant_warn.poshash then
+				technic.redundant_warn.poshash = true
+				print("[TECHNIC] Warning: redundant switching station found near "..minetest.pos_to_string(pos))
+			end
 			return
 		end