If someone has the same problem, I had to check if the TableCell elements already had a property. To check the elements I worked with Tags.
This is the code I used to check if TextField already was set or not:
var setTextField = cell.ViewWithTag (99) as UITextField;
if (setTextField == null) {
TextField = new UITextField ();
TextField.Placeholder = element [indexPath.Row].Placeholder;
cell.AddSubview (TextField);
} else {
//Nothing
}