Add data to Multi-select field

So are you looking for the original fields to be "de-duplicated" and listed vertically in the resultant field?

No, there is not a way to filter that.

An alternative is to go to a proper Many to Many table set up where you add multiple child records which then can be rolled up into the parent if you want.

There is an app I put in the exchange called User Focus Examples and that does a demo of how the Quick Select can work to allow users to quickly select the children for a satisfying user experience. An advantage of the quick select method is you launch the user into a report which can be filtered however you like so that the choices offered are relevant 

Or contact me directly for a demo.

What I did in my app was:
1.    Create Relationship and lookup field which will give you option to select 1 option at the time - [Brand] (Text lookup field pulled from a separate table)
2.    Create Field which will Capture all values selected in field Lookup field Brand - Example: [Brand_Text]
3.    Create formula field which will check for already selected values and add new selected from [Brand] to [Brand_Text].
Example: [Brand_Formula] with code
If(Contains([Brand_text],ToText([Brand])), ToText([Brand_text]), ToText([Brand_text])="",ToText([Brand]), ToText([Brand]) & ";" & ToText([Brand_text]))
4.    Create From rule which will copy value from selection to [Brand_Text] (See Below)
5.    If you want to still have "Multi-select field" then you can use Pipeline to copy value from field [Brand_Text] to "Multi-select field"