This should not be possible. Give the IH06 output screen showing these 2 line items with M-Z065 as input into selection screen F/Locn. Quite pissible that you are ignoring some difference in these two labels, which you are treating as identical. (Like a space or a dot at the end etc). Also give the structure indicator.
See this example
ABC-ACV and ABC- ACV both can be created. Notice there is a space before ACV in the second label.