Remove duplicate text from a cell

I am using excel 2010 and need a formula to remove the duplicate text from a cell.
In my excel the cell B1 is a list box with a list of text like "Apple, Banana, Grapes". When the user selects Apple, I have a formula in A1 which concatenates the text with whatever A1 has currently.
In the starting A1 will have nothing. when user selects Apple in B1, A1 changes to "Apple", if the user selects again "Apple", the A1 is having "Apple,Apple". I wanted the A1 cell to have unique text always sorted alphabetically.

Any help is appreciated.

Thanks

This sounds like circular referencing!
From what cell will the user be selecting "Apple" the second time?

Yes, its a circular reference. The Apple will be selected again from the same B1 cell.