I have an cross forest Exchange environment where Domain A is running Exchange 2010 and Domain B is running Exchange 2013. We are in the beginning of migrating Domain A Exchange users to Domain B Exchange. One issue I have seen is when a user in domain B creates a meeting request for someone in Domain A, when the user in Domain A accepts the meeting request there is a double entry on the Domain B's calendar event, one entry for the GALsync and one for a user with Domain A's email address.
If the creator of the event, the user in Domain B makes a change to the meeting that does not require acceptance from Domain A the event merges the Domain A's entry on the event to show just one user. However, if the creator makes any other changes that require the Domain A user to accept the double entry comes back.
Basically is seems that when ever a message - accepting a meeting invite is received from domain A then the Exchange server on domain B does not see it as the GALsync contact and adds it to the event as a new users.
GALsync is working without error, Free-Busy lookup is working as well.
Thanks in advance.