If you copy/paste values into the segments/individual target field, it allows you to paste duplicate values. (If you tried to type it, it wouldn't allow it to enter.) The only error they get is a vague error message with no details. If you have long lists of values, it would be difficult to know what the issue is. The ask is to either improve the error messaging to help identify the issue, or not allow duplicate values to be pasted in at all.