Only one group is remained in To field, other groups will disappear when duplicated members exist in multiple groups

Reproduce steps:

1. Create a group named group1, filled members, for example user1, user2, user3, user4

2. Create another group named group2, filled members same as group1.

3. User5 send a mail to group1 and group2

Result:

User1 open this message, he only see group1 display in To field. Can NOT see the group2 in TO field. This is work as design since they have the same members, router only sent the message once after namelookup.

Request:

End users sometimes misunderstand they couldn't send to the dropped group (group2) and send to the group(group2) again.
Request the group (goup2) remain or new setting to keep the group(group2) will be implemented after sending to such groups.