The value of a mitzvah may indeed be increased by its repetition. For example, the observance of Shabbat has clearly been of greater value to the survival of the Jewish people than the observance of Yom Kippur. In fact, a perfect test case for this question would appear to be Yom Kippur that falls on a Shabbat. It seems that here the priority is reversed, i.e. Yom kippur "docheh" Shabbat. At the same time YK is the Shabbat of Shabbatot so perhaps it intensifies rather than supersedes Shabbat.
Yet the blessing over the candles is still "ner shel Shabbat veshel Yom Hakippurim".
Shabbat shalom from Eve and Rabbi Rosenberg
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