Contribution plugin and Simple Vocab - adding multiple data to the same field

I have contributions and simple vocab working well together. I have one thing I'd like to do that I can't figure out if it's possible.
I have one field "Marks" that uses simple vocab and it has a list of 100 types of marks. In the admin view-Add Item I can add a number of types of marks to the "Marks" field.
I'd like to offer contributors the opportunity to enter more than one mark in that field using the drop-down menu. I tried adding more than one of the "Marks" element in the "Contribution | Types | Edit" thinking that I could just make the field repeat a few times on the contribution form, but the extra lines of the same element disappear from the contribution type when I save the form and thus never show up on the public contribution form.
Any thoughts or ideas would be appreciated!