Listbox nightmare

[dma431]

Hi,
I've been trying to figure this out for a while but my VBA is not at that level.
I have a multi-selection listbox with the checkboxes (fmListStyleOption) that feeds it's input to a cell. If I open the listbox again, the input will be added to the existing value of the cell.
The problem is, if I select an item in the listbox, that already exists in the cell, then I end up with a duplicate.
What I wanted to accomplish is for the listbox to check the existing cell values and put a checkmark next to those items, so only new items can be added.
Does that make sense? Is it possible?
I would really appreciate the help. Thanks.

 

[CSmith]

a sample of data with what you already have in the cell would be good to see. You really just need 2 sets of code attached to the listbox. Is the listbox on the sheet or a userform?