add filter for duplicate user/amount/date transactions

[tolland]

reconcile.rb relies on the OFX transaction data field FITID to determine the uniqueness of a user payment (for payments/transactions imported from bank statements, i.e. not gocardless) across multiple imports.

It would be useful to be able to support multiple ways of importing bank transaction data, e.g. open banking API transactions data. In that case the FITID wouldn't be available to match on, and so payments would be registered multiple times if both types of payment processing were run redundantly.

This change adds a filter so reconcile only adds the first payment it sees for a user, of a specific amount, on a particular day, regardless of the FITID associated with the payment.

Pros

• can run both reconcile and nordigen_import at the same time for redundancy
• users don't generally make more than one payment of the same amount per day, so this is an edge case
• even if they did, it wouldn't have any affect on their memberships subs status as long as at least one was sufficient on its own

Cons

If a user for example pays £5 into LHS bank with their hackspace id as a reference say 3 times. Only the first payment would be registered against their membership