This is the 11256th most frequent Russian word.
двукратно
Twice or twofold.
Here, 'двукратно' is used as an adverb to express that something has increased by two times.
Он двукратно увеличил свои доходы в этом году.
He has doubled his income this year.
In this sentence, 'двукратно' indicates the action of verifying was performed twice.
Результаты эксперимента были проверены двукратно.
The experiment results were verified twice.
'Двукратно' here suggests that the rewards matched a multiplication by two.