Is it better to repeat a caption or reference an identical caption?
It depends on several factors:


Can you expect readers to read this part of the caption only once and remember everything important for the rest of the reading? Or do you expect that readers need to constantly consult the caption to understand your figure?
Do you expect figures to be read in order? Or is it likely that a reader will look at a later figure in the series first?
How much information does the caption contain? This is not the same as the first point, as not all of this information needs to be important. Also, this is not exactly about length; a short caption with a lot of numbers is worse than a long caption describing straightforward concepts.
Do you need the space used by the repeated captions?


Positive answers to the respective first questions are in favour of referencing.

But why reference at all, unless you need the space? Because redudancy, in particular in technical aspects is annoying and time-consuming for the reader. If a caption references another caption the reader still remembers, this is the quickest way of communicating it. Otherwise the reader has to read the same (usually boring) text again. Moreover, if there is an unneeded redundancy, I would expect that there is a reason for this, e.g., that you changed some details of what is described in the caption. This would usually make me frantically compare the two caption (and thus flipping back and forth).

If you have the space, I suggest a third way that takes the best of both worlds:


  Fig 7. 2×167 in 10–200 mM KCl: (a)–(c) as labeled in Fig. 6, namely: (a) shows the reduced intensity for the varied sample conditions with the inset showing a key region. (b) shows how the scale applied to each data as a function of the concentration. (c) shows the time dependence.


This way, all the information is available for the reader who does not remember or has never read the earlier figure’s caption. But the reader who did, knows that the rest of the caption is redundant.