I have got a List in c# (in Unity) and I read some data from this list, if the object with the data I want is not in the list, I create the object and save it.

If two characters want the same object, the first one does not find it in the list, so it saves it in the list, but right before it does that, the second character looked in the list, did not find the object, and saves the object again: the same object would have been saved twice. How could we prevent this?