private Set<ComponentPlaceholder> getAllActivePlaceholders() {

		Set<ComponentPlaceholder> set = new HashSet<>();
		Iterator<PlaceholderSet> placeholderIterator = ownerToPlaceholderMap.values().iterator();
		while (placeholderIterator.hasNext()) {
			PlaceholderSet placeholders = placeholderIterator.next();
			set.addAll(placeholders.getUsedPlaceholders());
		}
		return set;
	}
Look for placeholder with the same name and group.  If such a placeholder exists, then
there is already at least one instance of this provider showing and we will stack the
new provider on top of this one, ignoring any windowing positioning specified by the new
provider.
@param activePlaceholders the set of currently showing placeholders.
@param newInfo the placeholder for the new provider to be shown.
@return an existing matching placeholder or null.
Ghidra/Framework/Docking/src/main/java/docking/PlaceholderManager.java