wallet: check a key image isn't already present when adding one

If it is, it points to reuse of a tx key, which isn't meant to happen.
If it does, a key image collision means that only one of those
outputs is spendable, so the wallet selects the larger amount,
unless that output was spent already.

This causes a discrepancy betewen reported received inputs and
payment total.

Since tx keys are 256 bits, this should never happen except if
done on purpose, or if a sender uses a bad PRNG.

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