In the King James Version, the James 5:16 seems to contain a little redundancy and/or truism:
James 5:16 Confess your faults one to another, and pray one for another, that ye may be healed. The effectual fervent prayer of a righteous man availeth much.
The redundancy is "effectual" and "availeth much." If it is effectual (in modern usage) then it means it works.
The apparent redundancy is a result of a slight semantic shift and an attempt to convey a Greek word in English.
The underlying Greek is:
Jas 5:16 εξομολογεισθε αλληλοις τα παραπτωματα και ευχεσθε υπερ αλληλων οπως ιαθητε πολυ ισχυει δεησις δικαιου ενεργουμενη
In this, the phrase in question is: "πολυ ισχυει δεησις δικαιου ενεργουμενη"
πολυ => greatly
ισχυει => enables/has force
δεησις => (a / the) prayer
δικαιου => righteous
ενεργουμενη => [(the thing that) is (itself) empowering]
The tricky word, as you can guess, is the last one - it is a present middle participle, which is a bit challenging to express in English. The KJV translators tried to express its meaning using the phrase "effectual fervent." The point of the passage is that we should pray for one another, and that we should have confidence to pray for one another on the basis that an empowered prayer by a righteous man can accomplish great things, both as to physical healing and also conversion: with Elias' prayers for and against rain provided as an example.
Pray to God and pray boldly, for if God gives your prayer power, you may save a soul by prayer!