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Why do the first and second ionizations of a diprotic acid produce the same hydronium ion concentrations in solution?

I was going through Oxtoby Chemistry when I got confused by the following passage in the textbook. It said that "two simultaneous equilibria are involved in the ionization of a diprotic acid such as carbonic acid":

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I understand the chemical species for both the first and second ionizations barely dissociate, which means the hydronium ion production is very little.

However, what makes hydronium ion production identical (i.e. important point #1)? Shouldn't the second ionization occur to a lesser extent than the first ionization? Thanks.