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code/game/objects/effects/beam.dm

@@ -213,7 +213,7 @@
 	if(istype(spawn_by,/list))
 		sources=spawn_by
 	else
-		sources.Add(spawn_by)
+		sources |= (spawn_by)
 
 	if(_range==-1)
 #ifdef BEAM_DEBUG

code/modules/optics/mirrors/mirror.dm

@@ -83,7 +83,7 @@ var/global/list/obj/machinery/mirror/mirror_list = list()
 
 /obj/machinery/mirror/beam_connect(var/obj/effect/beam/emitter/B)
 	if(istype(B))
-		if(B.HasSource(src))
+		if(B.HasSource(src) || src == B.source)
 			return // Prevent infinite loops.
 		..()
 		powerchange_hooks[B]=B.power_change.Add(src,"on_power_change")
@@ -140,7 +140,7 @@ var/global/list/obj/machinery/mirror/mirror_list = list()
 				continue // Prevent infinite loops.
 
 			// For recursion protection
-			spawners += B.sources
+			spawners |= B.sources
 
 			var/beamdir=get_dir(src,B)
 

code/modules/optics/prism.dm

@@ -118,7 +118,7 @@ var/list/obj/machinery/prism/prism_list = list()
 				continue // Prevent infinite loops.
 			// Don't process beams firing into our emission side.
 
-			spawners += B.sources
+			spawners |= B.sources
 			beam.power += B.power
 			var/beamdir=get_dir(B.loc,src)
 			overlays += image(icon=icon,icon_state="beam_arrow",dir=beamdir)