Steps followed to test the Patch #27 on version 8.4.x:-
1- Applied the patch....

Steps followed to test the Patch #27 on version 8.4.x:-

1- Applied the patch.
2- Checked "Prevent duplicates" checkbox on edit vocabulary page.
3- Added two taxonomy referenced fields referencing same vocabulary in a content type.
4- Checked "Create referenced entities if they don't already exist" for both the fields in the respective content type.
5- Created a content for the above mentioned content type and selected same term for both the taxonomy fields.
6- Saved the node.

This still creates two terms with the same name under same taxonomy vocabulary.
Hence, this patch doesn't work as intended.

Please mention if i need to follow any other step too, if i missed something.