If you export to PDF and put zoom to 1600% the space between the double lines can be seen.
The file content.xml said: style:border-line-width-left="0.02mm 0.04mm 0.02mm
, that is 0.08 mm, or nearly 0.23 pt.
Printing 0.08 mm at 300 dpi will leave a 0.94 dot for both lines (and space).
EDIT: Setting line width to 9pt (the maximum) these are the values in content.xml.
![image description](https://ask.libreoffice.org/uploads/asklibo/original/3X/3/6/3656be8a9cbc4452715dcd4148fb0df3f7fab41b.png)
EDIT 2: Setting the line width to 6pt and 3pt bring some surprises: the proportions of double and thin-thick medium gap remains the same.
![image description](https://ask.libreoffice.org/uploads/asklibo/original/3X/2/5/2509ae4f94321edbdded41748f353f2c0ec51c76.png)
See edited source file (border line styles.odt).
Tested with LibreOffice 6.4.7.2 (x86); OS: Windows 6.1.
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