Note 5 / Android 6.01 Stock E-Mail - Sends Two Copies on Reply All

[garak0410]

I currently have a single user with a Note 5 with Android 6.01. He uses the stock e-mail app. When he uses reply all, everyone gets a duplicate email from him. This only started since he went to 6.01. He has always used the stock e-mail. Our email provider is Exchange Online.

I've combed through the settings and can't seem to find a cause to the problem.

Any suggestions?