John Wesley on Psalms 41:13

Amen - Signifies an hearty assent and approbation, and withal an earnest desire of the thing, to which it is annexed. And as the psalms are divided into five books, so each of them is closed with this word; the first here: the second, Psal 72:19, the third, Psal 89:52, the fourth, Psal 106:48, the last in the end of Psal 150:6, the doubling of the word shews the fervency of his spirit, in this work of praising God.