First of all, you should go to
/etc/apt/sources.list.d and remove any file which has not a filename ending with
Then you should edit the file
jitsi-stable.list and ensure that it has a single line. Most probably, you have several of them with conflicting values for
Signed-by (which may well simply be that one line has
Signed-by and another similar line does not).
Instead of editing
jitsi-stable.list you can also try doing this:
echo 'deb https://download.jitsi.org stable/' > /etc/apt/sources.list.d/jitsi-stable.list
but this time using a single
apt update should work.