Both issues are fixed with Mp3tag v3.00. The duplicate ACOUSTID_ID gets removed if you reset the list.
[2020-01-15] FIX: unspecific error message after add field dialog at extended tag dialog looses focus while no field is selected.
[2019-11-11] FIX: removed duplicated entry forACOUSTID_IDfrom list of standard tag fields.
Thanks for reporting!