Avoid blocking v-backup-user because of already dumped db

Well, in one db.conf I found duplicated entry for the same database.

root@btvc:~# cat /usr/local/vesta/data/users/btvc2011/db.conf
DB='btvc2011_cart' DBUSER='btvc2011_9091dn' MD5='*' HOST='localhost' TYPE='mysql' CHARSET='UTF8' U_DISK='1' SUSPENDED='no' TIME='' DATE=''
DB='btvc2011_cart' DBUSER='btvc2011_admin' MD5='*' HOST='localhost' TYPE='mysql' CHARSET='UTF8' U_DISK='0' SUSPENDED='no' TIME='' DATE=''
DB='btvc2011_cart' DBUSER='btvc2011_david' MD5='*' HOST='localhost' TYPE='mysql' CHARSET='UTF8' U_DISK='0' SUSPENDED='no' TIME='' DATE=''
DB='btvc2011_cart' DBUSER='btvc2011_djn' MD5='*' HOST='localhost' TYPE='mysql' CHARSET='UTF8' U_DISK='0' SUSPENDED='no' TIME='' DATE=''
DB='btvc2011_cart' DBUSER='btvc2011_jd' MD5='*' HOST='localhost' TYPE='mysql' CHARSET='UTF8' U_DISK='0' SUSPENDED='no' TIME='' DATE=''
DB='btvc2011_cart' DBUSER='btvc2011_oscar' MD5='*' HOST='localhost' TYPE='mysql' CHARSET='UTF8' U_DISK='0' SUSPENDED='no' TIME='' DATE=''

Obviously different users has access to the same database.
I know that this is impossible to create in Vesta.
But @Skamasle thirt-party script for importing cPanels backup obviously CAN do that... and many people use that cPanel importer.

This fix will avoid databases that are already dumped.
Without this fix backup script get stuck on stdin waiting for:
gzip: /backup/tmp.4YlV9Yr4m5/db/btvc2011_cart/btvc2011_cart.mysql.sql.gz already exists; do you wish to overwrite (y or n)?

bin/v-backup-user

@@ -393,13 +393,15 @@ if [ ! -z "$DB_SYSTEM" ] && [ "$DB" != '*' ]; then
 
         dump="$tmpdir/db/$database/$database.$TYPE.sql"
         grants="$tmpdir/db/$database/conf/$database.$TYPE.$DBUSER"
-        case $TYPE in
-            mysql) dump_mysql_database ;;
-            pgsql) dump_pgsql_database ;;
-        esac
-
-        # Compress dump
-        gzip -$BACKUP_GZIP $dump
+        if [ ! -f "$dump" ]; then
+            case $TYPE in
+                mysql) dump_mysql_database ;;
+                pgsql) dump_pgsql_database ;;
+            esac
+
+            # Compress dump
+            gzip -$BACKUP_GZIP $dump
+        fi
     done
 
     # Print total