In the Brocho Asher Yatzer, why do we repeat the words Nekavim (Openings) and Chalulim (Cavities)? First we should look at why we say the brocho in the first place!
The reason we say Asher Yatzar is to Thank Hashem for our bodies working properly. But not only that, we also thank Him for the fact that because our bodies are working properly, we can serve Him. He created us for this purpose after all! We were formed with a guf (Physical Body) and a Neshama (Spiritual Soul).
As Rav Wolbe says in his sefer, Alei Shur that these are two separate entities in one box so to speak. Our job is to bring the holiness from the Neshama and use it to the good of the Guf! We say these words twice because we realise that just as the body has openings and cavities so too does the soul, and just like if our body was to not work perfectly we would not be able to serve Hashem even for just one moment, how much more so should we not be able to serve Hashem if our Soul was not working properly!
