If I am understanding properly, when the data is added to the table, it has some sort of date/time stamp, and you want the most recent record added ignoring all previous... If this is the case, this may be a simple solution...

Index on mailinglistid descending unique tag tmptag

Since the records will be in natural order, the date stamp I would assume is most recent with the most recent record. Therefore by indexing on the id descending it will use the last record first. Then by having UNIQUE, it will skip the rest of the same "mailinglistid".

Hope this is what you needed.