Visualforce Page:

<apex:page standardcontroller="Contact">
    <script>
        function DisableCtrlKey(e){
            
            var code = (document.all) ? event.keyCode:e.which;
            if (parseInt(code)==17){
                alert("Please re-type your email address");           
            }   
        }
    </script>
    <apex:form>
        <apex:pageblock title="Create Contact">
            <apex:pageblockbuttons>
                <apex:commandbutton value="Save" action="{!save}"/>
            </apex:pageblockbuttons>
            <apex:pageblocksection columns="1">
                <apex:inputfield value="{!Contact.Firstname}"/>
                <apex:inputfield value="{!Contact.LastName}"/>
                <apex:inputfield value="{!Contact.Email}"/>
                <apex:inputtext value="{!Contact.Email}" label="Re-Type Email Id" onkeydown="DisableCtrlKey(event)"/>              
                <apex:inputfield value="{!Contact.Phone}"/>              
            </apex:pageblocksection>
        </apex:pageblock>
    </apex:form>
</apex:page>

Output