It could be your mail server is having a glitch.
you have verizon if I recall correctly ??

You might try going to the Verizon website and find their email FAQ section or send them an email asking about it.
You can check your email on the Verizon website also and see if duplicates are there or not.

I haven't used Outlook express for quite awhile - I use Thunderbird from Mozilla.
For a short time I got a few duplicate emails and then those stopped after a week or so.

So I never had to figure out where the problem was coming from - I have Verizon also.