Let's divide the verse into two parts:
נשאו נהרות ה'—נשאו נהרות קולם
ישאו נהרות דכים
Each part is one sentence.
The first one is broken up poetically by a caesura, around which both the subject and verb are repeated. So the reason "נשאו" and "נהרות" are identical in each of their appearances is simply that they are repetitions of the same word....