Excel drop down list - deselect

Question

I have created a drop down list in Excel 2016. I have added it to a cell using the data menu/ data validation. It shows up correctly and allows the user to add multiple values (comma separated). The problem is when a user tries to delete any of the values in the cell (if picked by mistake). They double click in the cell and delete any unwanted values but when they move away from the cell, the values deleted come back twice. So if they start out with A,B,C,D and delete C - the result is A,B,C,D,C. Any help would be appreciated.