When you are creating a new object you can use a calculation to automatically generate the name. But if you leave the title field blank then you will get an error message saying that it is a duplicate entry. This is true even though the calculation would have put a unique value in there.
To fix this you need to setup a copy rule to copy any attribute to the title field. When you save the object it will initially use the copy rule to copy the data into the field allowing the prerequisite checker to pass. Then the calculation can fire off and put the correct value into the field and the object will save.