Comentario sobre Exodo 18:18

נבל תבל — Explain this as the Targum does: thou will certainly become weary. Its meaning expresses the idea of withering, old French flestre, just as. (Jeremiah 8:13) “and the leaf is withered (נבל)”; (Isaiah 34:4) “as withereth (כנבל) the leaf from off the vine” — the meaning being that it becomes shrivelled through the heat and through the frost, and so its strength diminishes and it becomes, as it were, weary, and falls from off the vine (מגפן)‎.

גם העם הזה אשר עמך, the member of your court.

נבול תבול, "you will surely become weary, etc." Yitro repeated the word נבול because it has two meanings. We apply this term to describe anything which has lost some of its original strength, and as a result has become weary, fatigued, relative to its original state. Nonetheless the subject under discussion is still able to perform its basic task though with greater exertion. The second meaning of נבול is a feeling of weariness which is felt so keenly that the subject is unable to carry out his regular routine at all. Yitro warned Moses that though, intially, he would feel only slightly weary, eventually he would become so weary, נבול תבול, that he would be unable to function at all. Yitro underlined this by saying: "you will not be able to carry on by yourself." Alternatively, the reason he repeated the word was simply because he wanted to illustrate how both Moses and the litigants would become weary.