Duplicates in recipient list when sending email to members of static group

This has only been happening in one group.

When I try to send an email to all/some of the members of a static group, their names show up duplicate (sometimes 5 or 6 times) in the recipient list.

Strangely, the number of times their names appear corresponds exactly with how many total groups the contacts are in (i.e. John Doe is in 5 groups total, so his name shows up 5 times in the recipient list; Jane is in 3 groups, her name shows up 3 times).

This isn't happening with other groups that I've tested.