The Append UI when appending multiple queries

I have dozens of queries that I combine together using Append in Power Query for Excel for Windows (Office 365). When adding a query from the left side (Available Tables) to the right side (Tables to append) the added queries still remain on the left side and can be added multple times. As a result, it's hard to determine if a table on the left side has already been added.

Is there a reason the dialog box works like this? Other Microsoft products that use a similiar style dialog remove the items from the left when added to the right. I can't see a reason why I'd want to add the same table multiple times. Even if I wanted to do so, it would be good if there was a toggle to "filter out added items" or something similar. At a minimum, even an indication showing which queries have already been added to the right side would be helpful.

There are a number of things that can be improved about this dialog.  Here's my list:

1. Adding items to the right side should remove them from the left side
2. Dialog box can't be resized. (Sometimes you just need a larger view.)
3. The list can't be alphabetized. (Being able to sort the list would make it easier to find things. Currently items are listed in the order that they are created.)

I think that changing these three things about the Append (and Merge) dialog would be very helpful. Thanks!