Elle,
according to our logs the first part (Enemy Appearance > Equip the Darkguard Armor) of Plans of the Secret Order quest has been completed successfully.

Just finished the same quest by myself and obtained / equipped all six parts without any issues.

If you can see my backpack you will note that I have two helms. When i equip the five pieces I do not get access to the final battle. Thus I cannot complete the quest.

Elle,
try to drop or sell one of them, then try to get the missing part of armor by slaying Darkguards few times. If that doesnt help let me know and I will see what I can do.

Ok, I cant drop these items, I killed about 30 Darkguard Killers and got nothing. I can't see Gamaldhar, Ashalkhan, or Kakhantur even with the 5 items equipped.

Elle,
I will get back to you as soon as it's fixed.

Elle,
the second Helm of Darkguard has been changed to Cuirass of Darkguard. You should be able to finish your quest now.

Much better thank you!