When refreshing the Org agenda without completing redrawing it (i.e, by calling
`org-agenda-filter-by-tag`), effort estimates where added for every refreshing,
causing them to stack up. We now check whether a summed effort estimate is
already present and remove it before adding it again.
(Maybe one could also just skip the effort recalculation if the a summed effort
estimate is already present, but I am not sure whether this will cause display
inconsistencies when new events are added or old ones are deleted.)