In the following article:
I demonstrated a method of obtaining unique items from a 3D range. At the time that this article was written, I was unaware that the TEXTJOIN function accepted native 3D references. See:
The techniques needed to convert a delimited string to a unique delimited string have already been discussed.
So, with the starting point of a delimited string obtained by using 3D reference with TEXTJOIN, the link above will then allow you to create the unique array or delimited string.
To be clear, first TEXTJOIN is used with a 3D range argument to make a delimited string. Then, that string is used in the formula construction described in the link above to make an array of unique items, which can be subsequently used to make a unique delimited string.
Remember again, that you have to have the Excel version included in Office 365 in order for the TEXTJOIN formula to work.