Right now it is possible to set the value of a member multiple times in a single create or change activity. Only the last value will be the value that will be actually set. When dealing with entities with a lot of members, this behaviour is really prone to cause errors. As one member could be defined to a certain value on top of the list of members, but changed to a different value at the bottom. This certainly would be the case if the number of members exceeds the viewable space and requires a scrollbar.
As a member can only be set once during an activity, it would make more sense to restrict the members (that are selectable under the new button) to those which haven't got a assigned value yet.
This has already been reported as a bug (#55380), but has been marked as a feature request.
In some cases I want to add strings to an attribute, in that case it is expected behaviour.
On this same subject; it's ambiguous what happens when you use the value of another attribute in the expression for the value and you change this other attribute in the same change action.
Or make the list sortable on attribute name or order in the domain model definition