Fixes SSchunks duplication cases (Stairs , Fallingoff) (discordia-spa…

…ce#8508)

* Update atoms_movable.dm

* Fixes for test.

* Update step_override.dm

* Update step_override.dm

* Update atoms_movable.dm

code/game/atoms_movable.dm

@@ -338,7 +338,11 @@
 		var/atom/oldloc = src.loc
 		var/olddir = dir //we can't override this without sacrificing the rest of movable/New()
 
+		// Movement has either failed by Bump(), or we get moved to a new Turf after entering
+		// Either way , both should count as failures, the move is not on the aimed turf after all -SPCR 2024
 		. = ..()
+		if(!. || loc != NewLoc)
+			return FALSE
 
 		if(Dir != olddir)
 			dir = olddir

code/modules/unit_tests/step_override.dm

@@ -11,8 +11,11 @@
 /datum/unit_test/step_override/proc/TestStep(type_to_test)
 	var/atom/movable/AM = allocate(type_to_test) // alloc spawns them at 20,20,1||20,21,1
 	var/turf/T = get_turf(AM)
+	var/turf/D = locate(T.x, T.y + 1, T.z)
+	T.ChangeTurf(/turf/floor)
+	D.ChangeTurf(/turf/floor)
 
 	. = step(AM, NORTH)
 	. = . && T.x == AM.x
-	. = . && T.y + 1 == AM.y // eh?
+	. = . && (T.y + 1) == AM.y // eh?
 	. = . && T.z == AM.z