Note
You should probably search for occurrences of the ID in link elements and check to make sure that the links go to the correct topic.
Sandcastle Help File Builder Documentation
BE0058: Two conceptual content files have the same ID ({0})
Error BE0058: Two conceptual content files have the same ID ([id]). The file with the duplicate ID is '[file]'
Cause
This error occurs if two conceptual content topics have the same ID. This can happen if you copy and paste a topic to create a new one but forget to give the new file a new unique ID.
Solution
Each conceptual content topic file must have a unique ID. Locate the topics with the duplicate ID and change one of them by giving it a new unique ID value.
Tip
If you need to clone a topic, it is usually best to add a new conceptual content topic so that it gets a unique ID and then copy and paste just the content from the introduction element through the relatedTopics element from the source topic into the new topic.