A list box shows only distinct values. When you see apparently duplicate lines, there must be a difference. Either the field values are pure text values, then they might differ in some 'invisible' chars, like spaces.
You can create an expression in your list box
to see if the length is different for the lines.
Or the values have an underlying numeric representation that differ (like a QV date).
That's what I assume, that you have date or timestamp values formatted as Month/Year. Add an expression to your list box to check this:
If my assumption is correct, it may be worth considering keeping the more granular value in your data model. And if the values show a numeric representation, you can directly use QV date functions like month() and year() without the need to interpret the value using Date#().