Fixes arrivals duplicate checking (#24946)

The check should be done BEFORE the docking_port is initialized

code/modules/shuttle/arrivals.dm

@@ -26,8 +26,6 @@
 
 	preferred_direction = dir
 
-	..()
-
 	if(SSshuttle.arrivals)
 		WARNING("More than one arrivals docking_port placed on map!")
 		qdel(src)
@@ -35,6 +33,8 @@
 
 	SSshuttle.arrivals = src
 
+	..()
+
 	areas = list()
 
 	if(latejoin.len)