Duplicate terms created when multiple images uploaded

Problem/Motivation

When uploading multiple images with DropzoneJS it will create duplicates of each term on subsequent runs - the first image's terms are correct, the second image's terms are duplicated once, the third image's are triplicated, etc.

Proposed resolution

Fix the logic so that the correct terms are created.

Remaining tasks

Fix the logic so that the correct terms are created.

User interface changes

The bulk uploader (DropzoneJS) doesn't create duplicate terms.

API changes

n/a

Data model changes

n/a