Prevent duplicate dirs in /etc/exports [GH-2871]

Consider the case where you export the same directory on the host
to different directories in the guest, such as the following:

    config.vm.synced_folder ".", "/vagrant", type: "nfs"
    config.vm.synced_folder ".", "/data", type: "nfs"

Vagrant was creating `/etc/exports` entries like this:

    "/Users/username/project" "/Users/username/project" 172.16.93.129 -alldirs -mapall=503:20

By removing duplicates while processing directories being exported, Vagrant will export this instead:

    "/Users/username/project" 172.16.93.129 -alldirs -mapall=503:20