According to Sifsei Chachamin, the verse repeats the name of the holiday because Purim is mentioned in Megillas Esther by name five times. One reason for this is that there are five unique mitzvos of the day: matanos la’evyonim, mishloach manos, the public reading of the Megillas Esther, the feast, and possibly the saying of “al hanisim” in our prayers. As mentioned earlier, the Mishna (Megilla 1:1) teaches that there are a total of five days when it is possible to fulfill one’s obligation of hearing the annual public reading of Megillas Esther. However, it will not cease (be kept completely) for two of those days. For that reason, the word is written in its complete form twice.