Infinite loop when component is created inside onMount

[ghost]

Describe the bug

If a component is dynamically created inside an onMount handler, it triggers an infinite loop whereby onMount is called over and over until the app crashes.

Logs
Console will output Maximum call stack size exceeded.

To Reproduce
https://svelte.dev/repl/308f355e52e740b99834853a5edd6db8?version=3.17.3

Credit to @tivac for coming up with a repro.

Expected behavior
Component is created at target site without an infinite loop.

Severity
Seems pretty bad if you are looking to do this sort of thing.

Additional context

Spent a little bit debugging this and it looks like the component creation triggers a call to flush() which then calls the onMount because it hasn't been removed from the queue yet. The seen_callbacks set which is supposed to protect against recursion does nothing in this case because it is constantly reset to empty upon entry.

[ghost]

Actually, I have a fix for this.... lift the set declaration out of the function and call clear() at the end.

[Conduitry]

I believe this is a duplicate of #3218.

[ghost]

Whoops, yes it is, I'll close this and update the PR accordingly.