Once I drag a field from the Additional Fields pane on the left into the Group by or Aggregated Fields panes, it disappears from the Additional Fields pane. There are definitely times when I want to use a field more than once. For example, I might want to get the min and max of a field to later calculate the range or duration.
Right now, I have to abandon the Aggregation and go back to a previous step to copy the field I want to use more than once.
I'd love to see fields that are used stay in the Additional Fields pane (maybe lighten them or give some other visual cue that they've been used and maybe even a checkbox to hide them - but don't remove them!)