Check for duplicates

src/freedombone-app-jitsi

     # add jitsi repo
     jitsi_deb_repo=unstable
     apt-get -yq install wget debconf-utils
-    echo "deb http://download.jitsi.org/nightly/deb ${jitsi_deb_repo}/" >> /etc/apt/sources.list
+    if ! grep "jitsi" /etc/apt/sources.list; then
+        echo "deb http://download.jitsi.org/nightly/deb ${jitsi_deb_repo}/" >> /etc/apt/sources.list
+    fi
     wget -qO - https://download.jitsi.org/nightly/deb/${jitsi_deb_repo}/archive.key | apt-key add -
     apt-get update