My accounts package produces listings in form of CSV files. Typically the
listings are dates and (invoice)values. The dates are in order but non
sequential -in otherwords there are gaps. Some dates are repeated (think
'date, invoice value' where theres more then one invoice per day)

I regularly need to merge/consolidate two/more such listings into one so
that I end up with a dates list (without repeats) and value being the sum of
all invoice values for the (unique) dates referenced.

Note the dates and values differ from list to list but the dates are ordered
as above (increasing).