You are nearly in the territory we call "multiple items per condition" when
you do that - if you seach using e.g. Google groups for recent messages
containing the following words you should find some examples of how to do
that:

Peter Jamieson MERGESEQ

However, in this case it may be slightly simpler than the full "multiple
items per condition" thing. Try putting the following field at the beginning
of your line of fields:

{ IF "{ MERGESEQ }" = "1" "{ SET lDate "" }" "" }

Put the following field at the end of your line of fields:

{ SET lDate "{ MERGEFIELD mydate }" }

And in the column where you want the date field, put

{ IF "{ MERGEFIELD mydate }" = "{ REF lDate }" "" "{ MERGEFIELD mydate }" }

where
a. mydate is the name of the field in the data source that contains the
date
b. all the {} are the special field code braces you can insert using
ctrl-F9.