Unique elements inside each child element of XML based on condition

My requirement is that there should be only one listingagent TAG for each role_type Primary inside each commercial tag. 

<deal>
<commercial>
<party></party>
<party>
<role_detail>
    <role_type>Primary</role_type>
</role_detail>
<listingagents>
    <listingagent>1</listingagent>
    <listingagent>2</listingagent>
</listingagents>
</party>
<party>
<role_detail>
    <role_type>Secondary</role_type>
</role_detail>
<listingagents>
    <listingagent>1</listingagent>
</listingagents>
</party>
<party></party>
</commercial>
<commercial>


</commercial>
<commercial>


</commercial>
</deal>

If there are more than one I need to raise an error.

The code I tried is as follows 

var duplicates = doc.Descendants("Commercial")
                                    .Select(c => c.Descendants("listingAgent"))
                                    .GroupBy(c => c.ToString())
                                    .Where(g => g.Count() > 1)
                                    .Select(g => g.First())
                                    .ToList();


 if (duplicates.Any(c => c.Count() > 1))
{
   //show error
}
Jon Skeet
people
quotationmark

If your constraint is that there should be exactly one listingAgent element within each commercial element, that's very simple:

var broken = doc.Descendants("commercial")
                .Any(c => c.Elements("listingAgent").Count() != 1);

This doesn't ensure that elements are unique though. For example, given that requirement, this XML is fine:

<deals>
    <commercial>
       <listingAgent>1</listingAgent>
    </commercial>  
    <commercial>
       <listingAgent>1</listingAgent>
    </commercial> 
    <commercial>
       <listingAgent>1</listingAgent>
    </commercial>  
<deals>

Each commercial element here has exactly one listingAgent child element. They've all got the same value, but that isn't part of the stated requirement. If you want uniqueness, that's a different requirement - and one that could be independent of the number of listingAgent children. For example, this meets uniqueness but not the "exactly one child" requirement:

<deals>
    <commercial>
       <listingAgent>1</listingAgent>
       <listingAgent>2</listingAgent>
    </commercial>  
    <commercial>
    </commercial> 
<deals>

people

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