Unify send pay invoice behavior

Each LN node behavior differs when attempting to pay an invoice that was previously paid:

#### LND
Returns an error and the API returns a 409

#### CLN
Just returns the payment data without error.

The CLN implementation should check before attempting to pay the invoice if it is already paid and return a 409.

Start by locating the CLN implementation that handles paying invoices and any tests covering repeated payments. Compare its behavior with the LND path described in the issue; done means an already-paid invoice through CLN produces the same 409 API response as LND.

Written by the indexing model from the issue text.