There was a recent checkin with the message:

fdo#51777: add a hack for 1 twip DOUBLE borders:
    
    Arguably such annoyingly thin double borders don't make much sense
    anyway, because they're essentially 2 hairlines with ~no space between,
    but unfortunately older LO versions are able to create them;
    since the refactoring in 2d045cdb69176b280812dda0b813371cf1ac72e2,
    which changed the BorderWidthImpl::Get* methods to return 0 due to
    rounding, they were ignored at least in the HTML import, which is a
    regression.
    So add a special purpose hack that essentially rounds up the first line
    to 1 but not the other lines so the visual result is a hairline single
    border.



I think it would be nicer if, upon opening a document, LO presented a dialog saying something along the lines of:

[dialog]
LibreOffice has some concerns about your document. Please address these if you can.

[scrollbox]
[entry 1]
Objects were found with double borders less than [convert twips to mm, * 100] 2% of a millimetre in width. These will not be visible and may constitute an authoring error. What would you like to do?

  (•) Remove borders (Recommended) [i think the default should be to clean up the document]
  ( ) Make no change [second option maintains document load‐>save integrity]
  ( ) Convert to single border of minimum width
  ( ) Increase border width to minimum for double border

[other entries for different concerns that aren't necessarily errors]
[/scrollbox]

[buttons]
Make No Changes                   OK
[/dialog]


The (Recommended) option is always number 1 in the radio group for that concern, and is pre‐selected. The "Make No Changes" button acts as if option 2 was selected for each concern.