這個是由於v-for迴圈裡，key值可能重複了，所以會報這個錯

     <div @click="selectDay">
                        <span v-for="(day,i) in previousMonth" :key="i" class="previousMonth"> {{ day }} </span>
                        <span v-for="(day,j) in monthDay[month - 1]" :key="j+'b'" v-bind:class="isActive(day)" class="currentMonth">{{ day }}</span>
                        <span v-for="(day,k) in nextMonth" :key="k+'c'" class="nextMonth">{{ day }}</span>
                    </div>
 

：key標記不同的值，防止出現重複的情況