Ok hear is a senario. Lets say time travel is possible. Jack and Jill discovered a way to travel to the future. Jack and jill are both In the same room. jack travels 1 year into the future. Jack would see his basement one year older still made up from the same matter it was made up from a year earlier. Meanwhile Jill is looking at the same basement. Two identical objects can not co exist if they did, they would not be the same.