Excel formula or VBS

I have a workbook that currently contains 10 sheets (June2015Report, May2015Report and so on). These sheets are exports from a bank, so they contain all the same information, except for the user field, which changes due to employees coming and going. My question is two fold.
I want a main sheet (MainSheet) which I am going to use to show data pulled from the other sheets.. such as total hours billed and things like that.  
The user column has multiple entries (due to different types of accounts) for each user, so Rsmith will show up 5 or 6 times.

 On the main sheet I want to pull all the users from each Report sheet and place them into say column B, but I also want to strip away any duplicates. I know how to obtain data from different sheets.. my issue is that I need to be able to have this script\macro\formula not only look at these first 10 sheets, but next month when I add July2015Report, it looks at that list of users as well.
So in short I am looking for something that will look at column B on all worksheets except for MainSheet and pull that data into column B on MainSheet and strip away duplicates.

Or am I going about this the wrong way?