I use LibreOffice 6.3.2.2 under Ubuntu 18.04 64 bits.
Since I installed Language Tool 4.7, when I preview a doc in Writer, the menu bar is double and still remains in double (even after preview).
When I uninstall Language Tool 4.7, all becomes normal.
So, what ?..
I didn’t try under Windows, and with another version of LibreOffice.
According to this page, it will be fixed in LibreOffice 6.3.3.