MDEV-26910 mysqld_multi starts same instance multiple times with the risk to crash database

a group name may be present in a file more than once.
use hash to deduplicate.

scripts/mysqld_multi.sh

@@ -620,7 +620,7 @@ sub find_groups
   my ($raw_gids) = @_;
 
   my %gids;
-  my @groups;
+  my %groups;
 
   if (defined($raw_gids))
   {
@@ -659,7 +659,7 @@ sub find_groups
         # Use $2 + 0 to normalize numbers (002 + 0 -> 2)
         if (not defined($raw_gids) or $gids{$2 + 0})
         {
-          push @groups, "$1$2";
+          $groups{"$1$2"} = 1;
         }
       }
       elsif (/^\s*!include\s+(\S.*?)\s*$/)
@@ -674,7 +674,7 @@ sub find_groups
 
     close CONF;
   }
-  return @groups;
+  return keys %groups;
 }
 
 ####