Problem calling a function only once

DaPOPs Use a timer to call end() after a certain time. All you need is one line of code:

get_tree().create_timer(.2).connect("timeout", end)

DaPOPs

var hurt_timer = 0.0
var one_time_effect = false

func start(Delta):
	if one_time == false:
		Hurt_box.create_hit_effect()
		is_active = true
		one_time = true
	if is_active:
		hurt_timer += Delta
		if hurt_timer >= 0.2:
			hurt_timer = 0.0
			one_time_effect = false
			end()
		Hit_box.collision_shape_2d.disabled = true

func end():
	emit_signal("end_state")
	is_active = false
	one_time = false

1 - you don't need so many bools.

DaPOPs call a function "Hurt_box.create_hit_effect()" but I want it to be executed only once

do you want the thing to run every time the timer ends or just once and start over when pressing a button?

run every tick: just run it with end.

if hurt_timer >= 0.2:
	hurt_timer = 0.0
	one_time_effect = false
	Hurt_box.create_hit_effect()
	end()

run once: don't reset the variable after setting it to true, set it back to false from OUTSIDE THE SCRIPT

func end():
	emit_signal("end_state")
	is_active = false
	#one_time = false

myScript.one_time = false

DaPOPs when you call the start() function again once end() has been executed, the hurtbox function can be called again.

but you are calling this thing every frame... it's doing what it's supposed to.
try putting the timer OUTSIDE the function and having it call the function instead:

if hurt_timer >= 0.2:
	hurt_timer = 0.0
	start()

edit:

xyz DaPOPs Use a timer to call end() after a certain time. All you need is one line of code:
get_tree().create_timer(.2).connect("timeout", end)

the timer has to run continuously, so use a timer node instead.