@michalek welcome to the forum.
You omitted to complete the field that asks what editor you use. However as "double-line" does not appear to be something that happens with the Markdown editor I assume that you are using just the Rich-Text Editor (RTE)?
This post suggests css that can be used to control the line spacing. From the looks of it though you would be using the RTE only as the css seems to cause the Markdown Editor's Render Pane to never display a blank line between paragrahs, no matter how many times the enter key is pressed.