You really can't. Splunk does not expose how things were matched. Now, for debugging purposes, if you want to be freakishly clever, you can do something like this:
Assuming Splunk has the regex library configured to allow for duplicate subpattern names in a single regex (and I assume they do, but don't know this for a fact), then you could extract the field named common_name either as "a+" or "b+" -- but in the "a+" case we would also extract "unique1", and in the "b+" case we would also extract "unique2". This is taking advantage of some oddities of the regex engine.
Fun fact: This is also a semi-reasonable approach to replacing some instances of FIELDALIAS and some edge cases for EVAL
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