I have repeat section with two dropdowlist. If the user in the first "dropdownlist" select items where started by "INV", in the second "dropdownlist" should only see items beginning with "02"
dropdown (lookup) filters are designed to work just on equality condition and whole source field value.
so you can not filter by substrings.
what you could do is to categorize items in source lists of both dropdowns.
so create a separate list of categories. add lookup fields into both above mentioned source lists. and finally configure categories - assign the same category to all the items that start with INV in one list and all the items that start with 02 in second.
with this setup you should be able to filter second dropdown by the same category as assigned to the item selected in first dropdown.