node.duplicate(7) fails to duplicate signals for specific ownership configuration

[adabru]

There were some issues with duplicating signals before:
#1888
#5405

Godot version: 3.2.1.stable

OS/device including version: Arch Linux

Issue description:
Duplicated node loses signal-connections of children. This happens only in the case that the duplicated node is not a scene and it has a descendant that is a scene with descendants that have signal-connections.

Steps to reproduce:
Open the minimal reproduction project. Click on button "node.duplicate(7)", then click on button "workaround duplicate(7)", then click on button "node.duplicate(15)". Each click duplicates the node $DuplicateMe in a different way and adds it to the main scene. The duplicated node contains a descendant whose "ready" signal is connected to a printout. The first button with the code node.duplicate(7) misses the printout. The output is:

--- Debugging process started ---
Godot Engine v3.2.1.stable.official - https://godotengine.org
OpenGL ES 2.0 Renderer: Mesa DRI Intel(R) HD Graphics 4000 (IVB GT2)

received ready signal!

┌─ $DuplicateMe.duplicate(7) ────────────
│ Original Scene2.owner: SceneMain
│ Original Scene2/Receive.owner: Scene2
│ Duplicate Scene2.owner: Null
│ Duplicate Scene2/Receive.owner: Null
│ incoming connections: []
└─────────────────────────────────────
┌─ workaround ────────────
│ Original Scene2.owner: DuplicateMe
│ Original Scene2/Receive.owner: DuplicateMe
│ Duplicate Scene2.owner: Null
│ Duplicate Scene2/Receive.owner: Null
│ incoming connections: [{method_name:_on_IDoEmit_ready, signal_name:ready, source:[Node:1340]}]
└─────────────────────────────────────
received ready signal!

┌─ $DuplicateMe.duplicate(15) ────────────
│ Original Scene2.owner: SceneMain
│ Original Scene2/Receive.owner: Scene2
│ Duplicate Scene2.owner: Null
│ Duplicate Scene2/Receive.owner: Scene2
│ incoming connections: [{method_name:_on_IDoEmit_ready, signal_name:ready, source:[Node:1403]}]
└─────────────────────────────────────
received ready signal!

The workaround precedes the duplicate(7) with a recursive own on the duplicated node $DuplicateMe:

  var nodes = $DuplicateMe.get_children()
  while len(nodes) > 0:
    var node = nodes.pop_front()
    node.owner = $DuplicateMe
    nodes += node.get_children()
  var duplicate = $DuplicateMe.duplicate(7)

I didn't compile godot yet, but a possible solution might add some mechanism to Node::_duplicate_signals(const Node *p_original, Node *p_copy) that changes p_original to any encountered scene-instance with the same owner as the node the duplicate call was made on before it calls get_child(i)->_duplicate_signals(p_original, p_copy); or something similar.

Minimal reproduction project:
Signal Duplication.zip

[Calinou]

For the record, the value 7 corresponds to the flags DUPLICATE_SIGNALS | DUPLICATE_GROUPS | DUPLICATE_SCRIPTS.