old mdadm daemon wasn't stopped during update from mdadm-0:2.6-alt2 to mdadm-0:2.6.1-alt2, both processes (from /usr/sbin/mdadm and /sbin/mdadm) are running even after `service mdadm restart'
Tried to be fixed in 2.6.1-alt3: service script checks mdadm binary at /usr/sbin (old location) and /sbin (new location), and calls stop_daemon with overridden PATH that forces handling of the appropriate binary.
Fixed in mdadm-2.6.1-alt3.1