Fix duplicating sub-scene may get two copies of internal node

[Rindbee]

Previously, internal node might be mistaken for hidden_root and be duplicated again. Exclude those internal nodes to avoid this case.

Fix #84803.
Bugsquad edit: Fixes #74406.

[KoBeWi]

Shouldn't this be determined by owner instead of internal children? I don't really get why the internal nodes even get duplicated.

Then again, I doubt anyone would be intentionally owning the internal children, so the fix is probably fine anyway.

[Rindbee]

The owner of internal nodes is usually not set, and the following code does not check if the owner is not set.

godot/scene/main/node.cpp

Lines 2516 to 2518 in 41365c6

	if (descendant->get_parent() && descendant->get_parent() != this && descendant->data.owner != descendant->get_parent()) {
	hidden_roots.push_back(descendant);
	}

Nodes added through tool scripts may not have an owner set. 🤔

[KoBeWi]

What I mean is that internal mode's only purpose is to "hide" nodes from the user (get_children() etc. ignores them by default). Duplicating instances should only be affected by node's owners, not their internal mode.

Nodes added through tool scripts may not have an owner set.

Then they don't appear in the scene tree. I think it's more expected (or at least consistent) if they don't get duplicated.

[akien-mga]

CC @KoBeWi - does this look good to go now?

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 4.2.2.