OK!, The workbook uploaded as a zipped file to the link below works with multiple sheets. As per my previous post, you will need to edit the
Select Case part of the code to insert the worksheets that will require the multi select DropDowns and also the ranges in each sheet that will contain the DropDowns.
If a selection is made that already exists in the DropDown then you will get a message box advising that it exists and to confirm to delete. If answer is "Yes" then it is deleted from the existing selections but if "No" then left as is. I am not aware of any other reliable method because one does not really know if the user inadvertantly selected an already existing value or if the selection is with the desire to delete..
All of the code is in ThisWorkbook module. No code in the individual worksheet modules.
Link to the example workbook.
https://1drv.ms/u/s!ArAXPS2RpafCtTR5CAv905fxgztj?e=usxnOu
Feel free to get back to me again if any problems with the code.