Deuteronomy 31:6 begins as an exercise in repetition, “חִזְק֣וּ וְאִמְצ֔וּ אַל־תִּֽירְא֥וּ וְאַל־תַּעַרְצ֖וּ”. It is a poem, albeit a short one, a call to war, a military ditty. It is supposed to make people ready and, hopefully, willing to cross Jordan and venture into the Promised Land without the trusted leader and father figure who has just announced the change in management for the whole enterprise.
As we all know, it is better not to change horses in the middle of the stream, but Moses wants the children of Israel to do exactly that! There is no other way to handle the situation, so he gives the pep talk twice. First to the entire camp and then specifically to Joshua in front of everyone else, repeating to him the words he has delivered to the children of Israel.
There is no specific ‘managerial’ instruction. Joshua is one of the people. He is not a super-human and can feel the infamous fear and dread just like the others do. Netziv in Haamek Davar expounds beautifully on the meaning of “dread”, arguing with Onkelos’s translation of it as “dismay”.
“Nevertheless, there is certainly a difference in the meanings: “Dread” is the breaking of a platoon, such that they give themselves up to their enemies, whereas “dismay” is the breaking of the heart…”
God is assured of the conviction and resoluteness of the Jewish people, just reminding them not to be afraid of the impending battles.