How do you track how often an ehash has been issued for the original share? I don’t think this is the correct way to think about it. If ehash is to be “used twice”, you just have a /swap that outputs two different units. One is the actual reward (on chain/ln/ecash) and the other is another ehash.
Is there a way to link an ehash to a share?
By whom? A mint knows which share to which ehash after redemption since the mint has. But the redeemer need not be the originator, as with other ecash.
How would this work if an ehash is split into representations of less difficult shares?
This is already covered and uses the keyset