Would you bother preparing something to help prove your reincarnation, or would you simply take advantage of whatever you retained (and prepared for yourself) from your past life?
How might you use this ability to help yourself and others?
Edit:
To clarify, suppose you have some vague memory of your past life in your reincarnated self that helps you along and allows you to take advantage of whatever your past self prepared for you.
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It is today, back in the 1500s when the church was the law and government and could do whatever they wanted with basically no repercussions, not so much.