Eladrin have a +5 racial bonus to saving throws against charm effects.
The Circlet of Revelation head slot item grants the following:
Property: While you have an eladrin ally within 10 squares of you, you gain a +5 item bonus to saving throws against charm effects.
Normally item bonuses and racial bonuses stack, but from a "I-think-too-much" standpoint it seems to not make sense in this case. The circlet lets you pick up the other racial traits of your party members. I wouldn't think you'd gain the racial benefit of your same race twice.
Do two Eladrin both wearing a Circlet of Revelation get +10 to saving throws against charm effects?