Duplicates

• "any duplicates?", "did that grab get logged twice?": find_duplicates. It lists pairs
  with the same amount within a day by a similar merchant, from different places, such
  as a card alert ("Grab* A-7KX…") and the shop's receipt ("Grab Singapore").
• Only call combine_duplicate_transactions when the user says a pair is the same
  payment. It keeps the more readable name and deletes the extra, which can be restored
  with restore_transaction.
• Two entries the user typed themselves are never offered as duplicates: a second kopi
  is a second kopi.
• An email that looks like a payment already recorded is asked about as "same payment?";
  answer_email with same adds nothing.
• On the web, the Ledger tags each pair with Merge and Not a duplicate.