Sysbench breaks on multiple table test with MariaDB 5.2.7 + Aria

I run a benchmark test on pitbull.askmonty.org with the following settings:
   - sysbench 0.5
   - number of tables: 24
   - table size: 2 000 000 rows
   - DB: MariaDB 5.2.7
   - Storage engine: Aria with TRANSACTIONAL=0
   - Workload: slightly modified oltp.lua so that it supports multiple tables and locking of tables for Aria (see attached file oltp_aria.lua)

Sometime after a few minutes of testing, an error occurs and breaks the benchmark.
Here is the error that breaks sysbench:
ALERT: failed to execute MySQL query: `UPDATE sbtest13 SET k=k+1 WHERE id=4097`:
ALERT: Error 1062 Duplicate entry '1001243' for key 'k'
FATAL: failed to execute function `event': (null)

The problem is that the column 'k' does not have a unique constraint. Here is the "show create table sbtest13" output:
CREATE TABLE `sbtest13` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `k` int(10) unsigned NOT NULL DEFAULT '0',
  `c` char(120) NOT NULL DEFAULT '',
  `pad` char(60) NOT NULL DEFAULT '',
  PRIMARY KEY (`id`),
  KEY `k` (`k`)
) ENGINE=Aria AUTO_INCREMENT=2000001 DEFAULT CHARSET=latin1 MAX_ROWS=2000000 PAGE_CHECKSUM=1 TRANSACTIONAL=0

This error may occur (or may not) randomly after a few minutes of testing and didn't happen on my local computer. The error occurred while testing with 1, 2, 4, 8 and 12 threads during a 10 minute test, and everything was OK with 16, 24, 48, 64 and 128 threads. Considering that pitbull has 12 CPUs with hyperthreading on: 24 virtual CPUs, this could be a clue (or just a coincidence though).

Here is an example call to sysbench prepare statement:
sysbench --mysql-host=localhost --mysql-socket=/home/vlado/Projects/MariaDB/temp/mysql.sock --mysql-user=root --test=/home/vlado/Projects/MariaDB/sysbench/sysbench/tests/db/parallel_prepare.lua --num-threads=24 --oltp-tables-count=24 --mysql-table-engine=aria --oltp-table-size=2000000 prepare

Here is an example call to sysbench run with 4 threads:
sysbench --mysql-host=localhost --mysql-socket=/home/vlado/Projects/MariaDB/temp/mysql.sock --mysql-user=root --test=/home/vlado/Projects/MariaDB/sysbench/sysbench/tests/db/oltp_aria.lua --oltp-tables-count=24 --max-time=600 --max-requests=0 --num-threads=4 --mysql-table-engine=aria run