Improve ProFormA compatibility between CodeOcean and CodeHarbor for multi-referenced files

[MrSerth]

When testing #1521, I noticed a stange behavior when exporting such an exersie with multiple references between CodeOcean and CodeHarbor:

• Importing task.2.0.zip in CodeOcean creates only one javatypesel file. When exporting this to CodeHarbor, only one test gets equipped with that one (in my tests, the last one).
• Importing task.2.0.zip in CodeHarbor creates three javatypesel files (one per test). When exporting this to CodeOcean, all three files get mirrored there, and end up as duplicates (same name, same path, same content, same attributes).

I don't know how easy these things are to fix. The deduplication (when exporting from CodeHarbor to CodeOcean might be easier, but might also require additional checks in CodeOcean [such as validating file paths / names and requiring at least different file roles for the same path]).