What is safek beguf safek beta'arovet? Specifically, what is the the safek beguf? Also, How does it relate to safek sefeka?
A concise explanation is brought in Halachapedia here:
If there is a safek safeka that is built of one safek about the status of the item in question and one safek since it was lost in a mixture, many rishonim consider this an invalid safek safeka. Others consider an acceptable safek safeka. One example is a safek teref piece of meat that was mixed up with other pieces of kosher meat (when bitul wasn't relevant).1 The halacha holds that it is an invalid safek safeka.2
If it is a safek safeka with one safek that is about the status of the item potentially a dvar sheyesh lo matirin and one safek of the tarovet it is an invalid safek safeka.3
1 Tosfot Beitzah 3b s.v. vacherot writes that Rabbenu Tam considers a safek safeka even if one safek is about the status of an item and one safek is about a tarovet. This is also the opinion of Tosfot Zevachim 71a and Riva in Shitah Mikubeset Zevachim 74a. However, the Ri cited by Tosfot disagrees that such a safek safeka is an invalid safek safeka. Rashba Beitzah 3a s.v. nitarva explains the opinion of the Ri; since the first safek is about the status of an item we determine that we are strict since it is a safek deoritta lchumra. Then we consider the second safek however at that time there's only one other safek. Rashba responsa 401 agrees with Rabbenu Tam.