There isn't an option like that.

I guess the refresh button in Edit Recipients does not do what you need,
either.

Typically, it is quite difficult to arrange that the list of records to be
merged is /only/ the list of records that have not been merged previously.
You either have to
a. get Word to select records based on when they were created - e.g., if
you were merging every hour, you might be able to specify records created in
the last hour or so (say). But then something has to be inserting a field
that says when the record was created. And you are probably never going to
get exactly the right records. Or
b. have a process that marks records as merged in your data source after
you have sucessfully merged them. But then you typically want to be
absolutely sure that you have merged them and do not need to re-merge them,
before marking them. And even that is not completely trivial - if you
automate Word to do it and it just updates your records as soon as it has
completed its output, there's always the risk that the output does not print
correctly (for example).