Based on Likutey Moharan lesson 24
The words found at the end of the description of the service of the Levi'im in the Mishkan, the verse uses term "Avodat Avodah" - the service of [another] service, in which Rabbeinu Bachyey explains as being a second Mitzvah found within every Mitzvah - i.e. the Mitzvah of doing the Mitzvah with joy, etc.
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