If you want to delete something on lemmy, make sure you edit the post/comment and replace it with something like "Deleted" then go to another instance to verify that the edit got sent to other instances. If you still see your old comment, then try editing it again later to a different text like "Deleted 2". Only click the delete button after you've verified that your original text is no visible from other instances.

Now why do I say this? Because if you delete the comment without overwriting it, the deletion request might not get sent through the fediverse and now someone on another instance is still able to view your post. You can't sent the deletion request twice, but you can edit a comment as much as you want, so if the federation is broken, you can always try to overwrite the comment again at a later time. Even then, there might still be a instance somewhere that didn't receive your update/deletion request, so please make sure you don't accidentally post your personal information.