How to avoid duplication of CSS classes

I am using less files for developing css but when I am compiling as css base.less get compile multiple times for eg:

style.less
   @import 'base.less'
   @import 'variable.less'
   @import '../../dashboard/my-dashboard.less'

my-dashboard.less
   @import 'base.less'
   .my-dashboard-css{
     all css goes here
}

when compiling style.less to style.css base css compiled twice in style.css Pls suggest me how to avoid me. Also Ihv tried @import(once) but its not working.

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