Prevent processing duplicate payment requests

Summary:
This is a backport of core#17031

Notes:

• I couldn't reproduce the duplication issue with our codebase, but the change makes sense nonetheless
• the deviation in the codebases is mainly due to D583

Test Plan:
ninja all check-all

Test that the payment request on the command line still works:
build/src/qt/bitcoin-qt -ecash ecash:qr5pfj5zqstsrf6qcqsh0w0yqr3h7gywuv759h27ss?amount=10

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9896