报错：“Duplicate keys detected: '2'. This may cause an update error.”

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这个是由于v-for循环里，key值可能重复了，所以会报这个错

     <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标记不同的值，防止出现重复的情况