I would like to preface this by saying the recent version update patch notes state that alter egos may be re-summoned in Legion even while having enmity on a target.

Platform: Windows
ISP: N/A
Type of Internet Connection: Cable
Internet Connection Speed: 30mbps
Date & Time: N/A
Frequency: Always
Character Name: Zaelle
Race: ElvaanM
World: Shiva
Main Job: Blue Mage, level 99
Support Job: NIN
Area and Coordinates: Hall of Ki
Party or Solo: Solo
NPC Name: None
Monster Name: Mired Khimaira, Mired Hydra, Mired Cerberus
Steps:
1. Entered Legion battlefield Hall of Ki.
2. Summoned alter egos Valaineral, Apururu, Zeid II, Ulmia, Koru-Moru.
3. Engaged in battle vs. Mired Khimaira, Mired Hydra, and Mired Cerberus.
4. Alter ego Apururu was defeated in battle.
5. Attempted to re-summon alter ego Apururu.
6. Game will not allow re-summoning Alter ego stating "Alter egos may not be summoned while having enmity on a target."