PluginMail fails sending to multiple groups

• Error

PluginMail fails when the address list contains two groups with users being members of both groups.

I tested with 3 groups:

Group10 (user11, user 12)
Group20 (user21, user 22)
Group30 (user11, user 12, user21, user 22, user31, user 32)
Sending mail to Group 10+Group20 works
Sending mail to Group 10+Group30 returns an error like

System error.

The following error message was returned:

The query was:
select `email` from `users_users` where `userId` in (?,?,?,?,?,?,?)

Values:

   1. 8
   2. 9
   3. 6
   4. 7
   5. 10
   6. 11
   7. 1

The built query was likely:
select `email` from `users_users` where `userId` in ('8','9','6','7','10','11','1')

Workaround 1:
A simple solution could be to address the groups one by one, however then some recipients will get the mail several times.

Workaround 2:
Try to edit file wikiplugin_mail.php
Replace in Line 267 in function wikiplugin_mail_to

$to = array_unique($to);

with

$to = array_values(array_unique($to));

This bug has been demonstrated on show2.tiki.org

Please demonstrate your bug on show2.tiki.org