Forbid two instances of a control in a repeat to have the same value

gistfile1.xml

<xh:html xmlns:xh="http://www.w3.org/1999/xhtml" xmlns:xf="http://www.w3.org/2002/xforms"
         xmlns:xs="http://www.w3.org/2001/XMLSchema"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xmlns:ev="http://www.w3.org/2001/xml-events"
         xmlns:xi="http://www.w3.org/2001/XInclude"
         xmlns:xxi="http://orbeon.org/oxf/xml/xinclude"
         xmlns:xxf="http://orbeon.org/oxf/xml/xforms"
         xmlns:exf="http://www.exforms.org/exf/1-0"
         xmlns:fr="http://orbeon.org/oxf/xml/form-runner"
         xmlns:saxon="http://saxon.sf.net/"
         xmlns:sql="http://orbeon.org/oxf/xml/sql"
         xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/"
         xmlns:fb="http://orbeon.org/oxf/xml/form-builder">
    <xh:head>
        <xh:title>No duplicate allowed</xh:title>
        <xf:model id="fr-form-model" xxf:expose-xpath-types="true">

            <!-- Main instance -->
            <xf:instance id="fr-form-instance" xxf:exclude-result-prefixes="#all">
                <form>
                    <section-1>
                        <grid-3>
                            <grid-3-iteration>
                                <choice/>
                            </grid-3-iteration>
                            <grid-3-iteration>
                                <choice/>
                            </grid-3-iteration>
                            <grid-3-iteration>
                                <choice/>
                            </grid-3-iteration>
                        </grid-3>
                    </section-1>
                    <section-5>
                        <control-6/>
                    </section-5>
                </form>
            </xf:instance>

            <!-- Bindings -->
            <xf:bind id="fr-form-binds" ref="instance('fr-form-instance')">
                <xf:bind id="section-1-bind" name="section-1" ref="section-1">
                    <xf:bind id="grid-3-bind" ref="grid-3" name="grid-3">
                        <xf:bind id="grid-3-iteration-bind" ref="grid-3-iteration" name="grid-3-iteration">
                            <xf:bind id="choice-bind" ref="choice" name="choice"
                                     constraint="not(../(preceding-sibling::*, following-sibling::*)/choice = .)"/>
                        </xf:bind>
                    </xf:bind>
                </xf:bind>
                <xf:bind id="section-5-bind" ref="section-5" name="section-5">
                    <xf:bind id="control-6-bind" ref="control-6" name="control-6"
                             constraint="count($choice) = count(distinct-values($choice))"/>
                </xf:bind>
            </xf:bind>

            <!-- Metadata -->
            <xf:instance xxf:readonly="true" id="fr-form-metadata" xxf:exclude-result-prefixes="#all">
                <metadata>
                    <application-name>community</application-name>
                    <form-name>no-duplicate</form-name>
                    <title xml:lang="en">No duplicate allowed</title>
                    <description xml:lang="en"/>
                    <singleton>false</singleton>
                </metadata>
            </xf:instance>

            <!-- Attachments -->
            <xf:instance id="fr-form-attachments" xxf:exclude-result-prefixes="#all">
                <attachments>
                    <css mediatype="text/css" filename="" size=""/>
                    <pdf mediatype="application/pdf" filename="" size=""/>
                </attachments>
            </xf:instance>

            <!-- All form resources -->
            <!-- Don't make readonly by default in case a service modifies the resources -->
            <xf:instance id="fr-form-resources" xxf:readonly="false" xxf:exclude-result-prefixes="#all">
                <resources>
                    <resource xml:lang="en">
                        <control-6>
                            <label>Invalid of duplicates above</label>
                            <hint/>
                        </control-6>
                        <choice>
                            <label>Choices</label>
                            <hint/>
                            <item>
                                <label>First choice</label>
                                <value>1</value>
                                <hint/>
                            </item>
                            <item>
                                <label>Second choice</label>
                                <value>2</value>
                                <hint/>
                            </item>
                            <item>
                                <label>Third choice</label>
                                <value>3</value>
                                <hint/>
                            </item>
                        </choice>
                        <section-1>
                            <label>Untitled Section</label>
                        </section-1>
                        <section-5>
                            <label/>
                            <help/>
                        </section-5>
                        
                    </resource>
                </resources>
            </xf:instance>

            <!-- Utility instances for services -->
            <xf:instance id="fr-service-request-instance" xxf:exclude-result-prefixes="#all">
                <request/>
            </xf:instance>

            <xf:instance id="fr-service-response-instance" xxf:exclude-result-prefixes="#all">
                <response/>
            </xf:instance>
            <xf:instance xxf:readonly="true" id="grid-3-template">
                <grid-3-iteration>
                    <choice/>
                </grid-3-iteration>
            </xf:instance>

        </xf:model>
    </xh:head>
    <xh:body>
        <fr:view>
            <fr:body xmlns:xbl="http://www.w3.org/ns/xbl"
                     xmlns:oxf="http://www.orbeon.com/oxf/processors"
                     xmlns:p="http://www.orbeon.com/oxf/pipeline">
                <fr:section id="section-1-control" bind="section-1-bind">
                    <xf:label ref="$form-resources/section-1/label"/>
                    <fr:grid id="grid-3-grid" bind="grid-3-bind" repeat="content" min="1"
                             template="instance('grid-3-template')">
                        <xh:tr>
                            <xh:td>
                                <xf:select1 id="choice-control" appearance="full" bind="choice-bind">
                                    <xf:label ref="$form-resources/choice/label"/>
                                    <xf:hint ref="$form-resources/choice/hint"/>
                                    <xf:alert ref="$fr-resources/detail/labels/alert"/>
                        
                        
                                    <xf:itemset ref="$form-resources/choice/item">
                                        <xf:label ref="label"/>
                                        <xf:value ref="value"/>
                                        <xf:hint ref="hint"/>
                                    </xf:itemset>
                                </xf:select1>
                            </xh:td>
                        </xh:tr>
                    </fr:grid>
                </fr:section>
                <fr:section id="section-5-control" bind="section-5-bind">
                    <xf:label ref="$form-resources/section-5/label"/>
                    <fr:grid>
                        <xh:tr>
                            <xh:td>
                                <xf:input id="control-6-control" bind="control-6-bind">
                                    <xf:label ref="$form-resources/control-6/label"/>
                                    <xf:hint ref="$form-resources/control-6/hint"/>
                                    <xf:alert ref="$fr-resources/detail/labels/alert"/>
                        
                        
                                </xf:input>
                            </xh:td>
                        </xh:tr>
                    </fr:grid>
                </fr:section>
            </fr:body>
        </fr:view>
    </xh:body>
</xh:html>