Email Signature stamped twice through Transport Rule when sending it to Distribution Group

• Looking for workaround/resolution for the following:
  Environment:
  Two Active Forests, one with Exchange 2007 and second with Exchange 2010. Migrating users and mailboxes from Exchange 2007 to Exchange 2010 by using ADMT and mailbox move. Two way trust relationship configured. Same domain name sharing between two exchange environment configured. domain is configured as internal domain. Internal mail routing between two exchange environments also configured.
  
  Everything is working fine except one ISSUE as below:
  In Exchange 2007 few users are member of one distribution group. Once we migrate the user to Exchange 2010 through Mailbox Move that user mailbox object converted in to "Mail User" object in Exchange 2007. (Until now everything is fine).
  In Exchange 2010 there is one dynamic distribution group and based on the criteria configured this migrated user become member of this distribution group. In Exchange 2010 there is one transport rule and this rule is used to create Signature at the end of each mail, if the user belong to that dynamic distribution group. (Until now everything is fine).
  Now if this migrated user send an email:
  1. to any user or other distribution group in Exchange 2010 ... Email Signature stamped correctly
  2. to distribution group in Exchange 2007 to whom he is a member-of ... Email Signature stamped twice (i.e; that email has Email Signature, but two times at the bottom) and every user in this group receive this email with two signatures...So, not worked correctly.
  3. to any user or other distribution group (to whom he is not member-of) in Exchange 2007 ... Email Signature stamped correctly
  
  The following workaround tried but non of them worked except if I remove the "remote user" object from Exchange 2007 distribution group then of course this user will not receive the email but other members of the group will receive email with single signature, which is correct.
  
  1. Removed Mail User object from the group in exchange 2007 and added a contact for the same user, but it was unsuccessfully.
  
  2. I have enabled the following logs, but no information in logs by simulating the issue:
  Get-EventLogLevel 'MSExchange Messaging Policies\Rules' | Set-EventLogLevel -Level High
  Get-EventLogLevel 'MSExchangeTransport\Agents' | Set-EventLogLevel -Level High
  Get-EventLogLevel 'MSExchangeTransport\TransportService' | Set-EventLogLevel -Level High
  
  3. Don't see any issue while analyzing message tracking or in email header.
  
  Any thought/workaround

Liaqat