efficiently removing duplicate xml elements in c#

I have couple of XML files that contain lots of duplicate entries, such as these.

annotations>
  <annotation value=",Clear,Outdoors" eventID="2">
    <image location="Location 1" />
    <image location="Location 2" />
    <image location="Location 2" />
  </annotation>

  <annotation value=",Not a problem,Gravel,Shopping" eventID="2">
    <image location="Location 3" />
    <image location="Location 4" />
    <image location="Location 5" />
    <image location="Location 5" />
    <image location="Location 5" />
  </annotation>
</annotations>

I want to remove the duplicate elements in the each of the child. The way I approached this is by copying all the elements to a list and then comparing them,

 foreach (var el in xdoc.Descendants("annotation").ToList())
   {
      foreach (var x in el.Elements("image").Attributes("location").ToList())
       {
           //add elements to a list
       }
   }

half way through I realized this is very inefficient and time consuming. I'm fairly new to XML, I was wondering if there are any built in methods in C# that I can use to remove duplicates?.

I tried using

if(!x.value.Distinct()) // can't convert collections to bool
    x.Remove();

But that doesn't work, neither does

if(x.value.count() > 1) // value.count returns the number of elements.
   x.Remove()