Fix duplicated words in comments

Summary: This is backport of PR#478 by Dimitris Tsapakidis

Test Plan:

make check

Reviewers: schancel, MattBitprim, #bitcoin_abc, Mengerian

Reviewed By: schancel, #bitcoin_abc, Mengerian

Subscribers: Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D784