The prose style is unscientific - 'arguable' would be OK in other contexts, non-scientific academic papers, science journalism etc. but not science proper. The writer might be expected to be 'loose' in other ways. Two identical control groups make no sense: a controlled experiment needs a minimum of two identical groups (control and experimental/treatment group). If it is not possible to create two identical groups, it is not possible to make a controlled experiment. This is what the writer is trying to say. All members of the control group must be treated identically, so splitting it in two can serve no purpose - if the two control groups were selected or treated differently in any way, that would break the protocol of the experiment, so why do it? Also, the statement as written is logically confused. If it is not possible to create two identical groups (control and experimental), that means it is impossible to create one control group. We can easily deduce from this the inability to create a second control group ('two identical control groups'), a trivial consequence unworthy of mention, certainly not the cause of the 'problem'. The writer says it is arguable that identical groups cannot be created, which implies that maybe they think they cannot be, or maybe they don't: 'arguable' simply means that it is reasonable to argue the case that they cannot. This tells us nothing, it is sitting on the fence as lawyers do, not scientists. Instead, the writer should make an assumption (can or cannot) and present a justification for it, the language here doesn't seem to be heading in that direction: 'we argue that' would be a good start..

1) "identical control groups", when conducting experiments you need to use 'subjects', the people/animals/data that you are testing. (EDIT, misread question slightly) In addition there can be a Control Group, a group who do not undergo the experiment at all. 2) "which, it is arguable, cannot be created.", this is stating the probable truth that it is impossible to create two completely identical control groups! Just imagine trying to find 2 identical English learners, let alone 2 perfectly identical groups of 300 learners! Other examples? Even comparing June 2018 to June 2019 or to July 2018, the comparison cannot be identical, something is a little different. Technically even you are a little different today than yesterday! I believe it is just an accepted fact and reality of conducting experiments using control groups for comparative purposes!