You notice that the recipients receive two copies of emails when the email is sent to a Microsoft 365 group (not a distribution or security group) and the email is being passed to Exclaimer for processing signatures.
This is a known issue with Microsoft.
The main cause of this issue is that with Microsoft 365 groups, the email is not 'dropped' when the message recipients are 'split' into the individual users from the group list.
Usually, with Distribution lists, the original 'group' email is dropped/deleted; instead, all the users have an individual email that is sent to Exclaimer for processing signatures.
With the Microsoft 365 group, the individual emails are sent, but the group mail is not 'dropped' and this is also sent as its own email to Exclaimer for processing.
Once Exclaimer processes the emails, it returns the individual user's emails and the group email back to the tenant. The individual mails are delivered as normal, but the group mail is processed again by Microsoft 365 - this is split once more into the individual user emails and sent to their mailboxes.
The way the email is processed, the duplicate email is not picked up.
Microsoft considers this as expected behaviour and does not offer a resolution, nor will this be fixed in future.
We advise you not to use Microsoft 365 groups; instead, use a distribution list.