OWL:SymmetricProperty not give the desired result

Question

I have similar it's owl:SymmetricProperty

<owl:ObjectProperty rdf:about="#similar">
        <rdf:type rdf:resource="http://www.w3.org/2002/07/owl#SymmetricProperty"/>
        <rdfs:domain>
            <rdf:Description rdf:about="#Protein"/>
        </rdfs:domain>
        <rdfs:range>
            <rdf:Description rdf:about="#Protein"/>
        </rdfs:range>
</owl:ObjectProperty>

I have 2 protein I put the first protein as similar of the second protein

<similar>
        <rdf:Description rdf:about="#protein1"/>
</similar>

When I use SPARQL

PREFIX ns:<http://www.owl-ontologies.com/unnamed.owl#> \n   
        SELECT ?p1 ?p2 WHERE{ 
        ?p1 ns:similar ?p2.};

I only get one result

-----------------------------
| p1          | p2          |
=============================
| ns:protein2 | ns:protein1 |
-----------------------------

But I want the following result

-----------------------------
| p1          | p2          |
=============================
| ns:protein2 | ns:protein1 |
| ns:protein1 | ns:protein2 |
-----------------------------