This value already exists in the list

Hello, 

I try to copy-paste a set new items to a list. The field was a "unique value" field before. But even turning it "unique value" off, didnt change its behaviour:
Each time I pasted new items the first of second item showed the error message "This value already exists in the list".

I tried different sets of numbers, the latest try was with a range of numbers, definitely not used before. I turned off "unique value", also still the same error. 

And bad impact of this is, after a refresh, all entries after the erroneous line are not saved. 

Any ideas? 

Best regards, 

Tonda