Sorry, it's been a long day and I need to look at this when I'm fresh but I did notice you are referring to one selected item in your formula, which is why you are getting the repeating result in the textbox.
Concat(ComboBox.SelectedItems, ComboBox.Selected.Result & ", ")
ComboBox.Selected.Result refers to one particular selected item, not a column from the table of results.
In this
Concat(ComboBox1.SelectedItems, Title & ", ")
"Title" refers to a column, not any particular selected item. You need to refer to a column in the reults of the selected items.
For example if you ComboBox records which you are selecting from contain the columns "ID", "Product", and "ProductValues" your formula may look like any of these;
Concat(ComboBox1.SelectedItems, ID & ", ")
Concat(ComboBox1.SelectedItems, Product & ", ")
Concat(ComboBox1.SelectedItems, ProductValues & ", ")