Check for existing dataset (same pid/id) while we have the lock.

apt/APT_Dataset.pm.in

@@ -182,6 +182,8 @@ sub Refresh($)
 sub Create($$)
 {
     my ($class, $argref) = @_;
+    my $pid = $argref->{'pid'};
+    my $id  = $argref->{'dataset_id'};
     my $uuid;
 
     if (exists($argref->{'uuid'})) {
@@ -208,6 +210,16 @@ sub Create($$)
 	return undef;
     }
 
+    $query_result =
+	DBQueryWarn("select uuid from apt_datasets ".
+		    "where pid='$pid' and dataset_id='$id'");
+
+    if ($query_result->numrows) {
+	DBQueryWarn("unlock tables");
+	tberror("Dataset $pid/$id already exists!");
+	return undef;
+    }
+
     my $query = "insert into apt_datasets set ".
 	" locked=now(),locker_pid='$PID', ".
 	" idx='$idx',created=now(),uuid='$uuid', ".

apt/manage_dataset.in

@@ -339,7 +339,7 @@ sub DoCreate()
 
     my $dataset = APT_Dataset->Create($blob);
     if (!defined($dataset)) {
-	fatal("Internal error creating dataset object");
+	fatal("Error creating dataset object");
     }
     
     # new dataset is returned locked. If we have instance, try to lock