i need the double overline in the libreoffice & there is double underline just , thanks for solving this problem .
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OS: Linux (All)
OS is 64bit: yes
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While there is no .uno button action, there are already multiple way to achieve this.
Most correct is to use the Unicode "U+033F COMBINING DOUBLE OVERLINE". Which--if the font you are using includes it--you'll have a the font designer's take on what is suitable to the font. If font does not have the glyph covered, you get a glyph from fall back font that may not be proportioned well.
Do this with the Unicode toggle feature enter each character followed by u+033f and toggle with <alt>+x. This can be done in an external Unicode enabled editor and strings pasted into LibreOffice.
A more general and easier method, but less pleasing (it does not consider internal leading) is a "font effect". There you type the text needed, select it, and then apply Overlining from the Character -> Font Effects -> Overlining. Default is "without", but select Double from the drop list.
Finally, also easy, apply a border to the selection. Character -> Borders. In the User defined widget click to add a line above. Then set its Line style from the list--the double line is the bottom selection. Also set a width.