如何去掉li标签的重叠边框

当我们的li标签设置了border的时候就会出现重叠边框，如何去掉呢，见代码

html代码

<ul class="friendLink_list">
                    <li><a href="javascript:;">营销费用管理</a></li>
                    <li><a href="javascript:;">营销费用管理</a></li>
                    <li><a href="javascript:;">营销费用管理</a></li>
                    <li><a href="javascript:;">营销费用管理</a></li>
                    <li><a href="javascript:;">营销费用管理</a></li>
                    <li><a href="javascript:;">营销费用管理</a></li>
                </ul>

css代码

.friendLink_list {
    list-style: none;
}

.friendLink_list li {
    float: left;
    border: 1px solid #aaa;
    display: block;
    width: 285px;
    height: 60px;
    text-decoration: none;
    margin:   -1px -1px;
    position: relative;
    z-index: ;
    text-align: center;
    line-height: 60px;
    font-size: 15px;
}

.friendLink_list li a {
    color: #b5b5b6;
}

设置最终效果图

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