Deduplicate XML using LINQ based on attribute value

I have a XML like this:

<rewriteMaps>
  <rewriteMap name="RewritesInSightContentaltecNL">
    <add key="/tata" value="/productdetails.aspx?id=1628" />
    <add key="/tata" value="/productdetails.aspx?id=1628" />
    <add key="/marc" value="/productdetails.aspx?id=1626" />
    <add key="/marc" value="/productdetails.aspx?id=1626" />
    <add key="/marc" value="/productdetails.aspx?id=1626" />

    <add key="/henk" value="/productdetails.aspx?id=162996" />
    <add key="/henk" value="/productdetails.aspx?id=1aa6" />

    <add key="/tata" value="/productdetails.aspx?id=1628" />
    <add key="/tata" value="/productdetails.aspx?id=1628" />
    <add key="/marc" value="/productdetails.aspx?id=1626" />
    <add key="/marc" value="/productdetails.aspx?id=1626" />
    <add key="/aaaa" value="/productdetails.aspx?id=xxxx" />
  </rewriteMap>

  </rewriteMaps>

From which I want to remove duplicate values base on the "key" attribute"

So I want to end up with:

 <add key="/tata" value="/productdetails.aspx?id=1628" />
 <add key="/marc" value="/productdetails.aspx?id=1626" />
 <add key="/henk" value="/productdetails.aspx?id=162996" />

Any help greatly appreciated.