I need a simplified UX solution for selecting many checkbox items (around 50) for export from within a modal window. The idea is that the customer would like to be able to chose multiple fields from a large database table to export in to a report. I don't want to just list 50 field names in a scrolling modal because it is difficult to remember what has been previously selected. The user would have to constantly scroll up and down to confirm their choices. I also don't want to shove all 40 field names in 3 columns in to a small modal window as it doesn't look very aesthetically pleasing.
This is one of the original reason that the dual pick list was created. When you have your source list on the left side a set of controls in the middle and the list of chosen items presented on the right. You keep everything in context for what is available and what has been chosen. Now if you add to this some of the more modern approaches for the source list like grouping by topic or alpha quick jumps you make thing even easier on the user. Adding an image for memory jog.
If there are logical categories you could consider grouping them to make them a little easier to find.
If there are no logical groups them perhaps consider organising them alphabetically like a Rolodex (I used the items in your screenshot to set the letter categories):
You can use styling to help the user identify which letter categories contain selected fields.