If you request list=allusers with a limit lower than the expected result count the last entry of the partial list is returned twice.

The limit can be the default limit or set by aulimit:
http://de.wikipedia.org/w/api.php?action=query&list=allusers&augroup=bot&aulimit=1

<query>
<allusers>

<u name=".snoopybot." userid="425305" />
<u name=".snoopybot." userid="425305" />

</allusers>
</query>
<query-continue>
<allusers aufrom="08-15-Bot" />
</query-continue>

Of the limit is higher than the result count the server response is ok:
http://de.wikipedia.org/w/api.php?action=query&list=allusers&augroup=bot&aulimit=400

This bug caused some wrong output of my bot on wikipedia today. On the last run on Februray 4th with MW 1.16 installed this was working correct.

Version: unspecified
Severity: major
URL: http://de.wikipedia.org/w/api.php?action=query&list=allusers