We have # of people using iPhone in my company, and finally this happen on one of the iPhone, and it happens on my boss's iPhone...
She only use the phone to check email at night - like when she is on the train and home.
And since a week ago, it starts reloading all her email from #1, I have 2 weeks only setting for her account because she has a lot of emails.
She is on iOS 5.x, iPhone 4S with Verizon.
By the way, it is working fine when she is at the office under wifi, but not on Verizon 3G.
What I will do - upgrade it to iOS6 after I come back from the vacation and see it can solve the problem.
We are hosting our own mail server with Microsoft Exchange 2003.