By mistake I posted an answer to Veselina's post instead of a comment and I found I posted 2 times the same answer.
Then I choose to delete the first one but it has been still there until I refreshed the page
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Yes, it is expected behavior. Because when you still on this page you can cancel your action and 'Undelete' your answer.
After refresh you can't 'Undelete' your answer.
It is done if you by mistake click 'Delete' button.
And how I can "undelete" it? I could not remember to have seen an "undo" button.
Should be in Actions->Undelete, the same place where you clicked Delete.