Godot: "Can't add child, already has parent"

I am trying to spawn a bunch of trees in a 3d space randomly.
Code:

extends Spatial

var position = Vector3.ZERO

onready var tree = preload("res://TREE.tscn")

var tree_amount = 400
var tree_count = 0
var can_add_tree = true

func _ready():
	randomize()

func _process(delta):
	position.x = rand_range(-200, 200)
	position.z = rand_range(-200, 200)
	
	var new_tree = tree.instance()
	
	if can_add_tree:
		for i in range(tree_amount):
			add_child(new_tree)
			new_tree.set_global_translation(position)
			
			tree_count += 1
			
			if tree_count == tree_amount:
				can_add_tree = false

However, when I run this it gives me the message "Can't add child TREE to Main3D, already has parent Main3D". How do I fix this?

With this line you create a new tree:

var new_tree = tree.instance()

With this line you add it to the scene:

add_child(new_tree)

The issue is that you are creating a single new tree and adding it to the scene multiple times in a loop.

Instead, create a new tree inside the loop. So each iteration of the loop is a new tree.

Addendum: Did you mean to add a new tree each frame (each time _process runs)? In that case, don't do a loop at all. On the flip side, if you want to create the trees at the start, you probably can run the code in _ready.