I tried to make clear that you have to execute the action a couple of times.
As I said: it splits 4 consecutive capital letters into 2 pairs.
So if you have GB;GBGB, then you simply have to run the action again to get GB;GB;GB.
And if there are 4 douplettes, you have to run it 3 times.
To remove duplicates, it would mean to first sort the list and then apply an even more complicated regular expression like e.g. from this thread: