Once a signature line is signed, it should not be possible to sign them again.
Moving to NEW
Samuel Mehrbrodt committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/core/+/7b6f477799d55d80775b6120ba0b5ce1bb45fb3c%5E%21 tdf#118582 Disable signature line signing once it is signed It will be available in 6.2.0. The patch should be included in the daily builds available at https://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More information about daily builds can be found at: https://wiki.documentfoundation.org/Testing_Daily_Builds Affected users are encouraged to test the fix and report feedback.