In the adding trustline list, currently it will hide the added trustline. My suggestion is, instead of hide it, how about gray out the option.
Use cases:
  1. I have owned CSC and I try to show my friend how to add CSC to their wallet. I can’t use my phone or wallet to showcase the CSC option on the adding list.
  2. When more trustlines available in the future, some users might forget they have already own it and when they it is gray out. They know the trustline is supported instead of struggling when to find it.