Which field to use for dedupe field on intermediary object?

I created a custom object "document" to host all the assets we have for download. And created a intermediary object "download" to connect document and lead. I linked download to lead with lead ID, and linked download to document with "document ID".

I have several fields on the download object, but none of them are unique identifier. What other fields can I create to use for dedupe?

• download datetime
• Download Source
• Created at
• Updated at
• Lead ID
• Document ID