in the attachment in bug 42750 there is another problem: the corners where the double borders meet look really ugly, the lines cross and stick out over the other border. this can easily be reproduced by creating a new table with double line style in Writer.
Created attachment 59908 [details] simple bugdoc the effect is interesting: the bug happens up to about 273% zoom level, but at 277% and higher the borders are connected properly and look right.
Michael Stahl committed a patch related to this issue. It has been pushed to "master": http://cgit.freedesktop.org/libreoffice/core/commit/?id=49bd0e4e6bb0ed0671de72d84700ddcc49828f69 fdo#48647: drawinglayer: fix double hairline borders:
so this was actually quite easy, the hairline case was not using the ClipRegion and simply clipping with the right parameters seems to result in neat borders. please note that because commit 2c91cb08d65cd35fa8ef6eaca3677aa82fb58cbe changed the hairline predicates on master such that a border in Writer is never considered a hairline (because Writer borders are never thinner than 1 twip), this bug cannot actually be observed in Writer on master now, but it can be observed in Calc on master.
Michael Stahl committed a patch related to this issue. It has been pushed to "libreoffice-3-5": http://cgit.freedesktop.org/libreoffice/core/commit/?id=339a502e1f78388512cc8b9ba2117b674c770efc&g=libreoffice-3-5 fdo#48647: drawinglayer: fix double hairline borders: It will be available in LibreOffice 3.5.4.