`_init()` called twice when extending script in inherited scene

### Tested versions

4.5 dev4

### System information

W10

### Issue description

When you make inheriting scene and extend the base scene's script, the `_init()` method of the base script is called twice.

### Steps to reproduce

1. Create a scene
2. Add a (non-built-in) script with `_init()` method that prints something
3. Create another scene inheriting that scene
4. Extend the script, can be empty
5. Run the inheriting scene

### Minimal reproduction project (MRP)

[initwice.zip](https://github.com/user-attachments/files/20504752/initwice.zip)
Inheritor.tscn is scene with bug.
NotInheritor is equivalent scene, but without inheritance. No bug.