[JIRAALIGN-1165] Able to add same objectives to an Epic

Issue Summary

User can add same Bbjective over and over again to an epic

Steps to Reproduce

1. Open an Epic
2. Scroll down to Objectives and add one.
3. Try to add the same objective

Expected Results

User should not be able to add the same objective again.

Actual Results

Users are able to add the same objective over and over again to an Epic.

Workaround

None.