Random Duplicate locations when using Google Maps for EE

Question

I've got Google Maps for EE set up to show a US map listing the locations of all of the firms within an organization. Everything seems to be working ok, but one of the locations is showing up twice (it's only entered once on the back-end). There are a few locations that are overlapping (making it hard to tell that), so I want to enable clustering to give a visual cue that more than one location might exist. However, when I do, I get other locations that randomly appear twice. Here is my code:

{exp:gmap:init id="member_directory" style="width:1000px;height:600px"}
{exp:channel:entries channel="firms" dynamic="off" status="open" limit="500"}
{cf_firm_location}
    {exp:gmap:marker
        id="member_directory"
        address="{m_address}{street}, {city}, {region} {postal_code}{/m_address}"
        zoom="4"
        //open_windows="false"
        //extend_bounds="false"
        //geocode="false"
        clustering="true"
        icon="/themes/third_party/gmap/icons/office.png"
        infobox="true" // Tells script to use infobox.js
        //offsetY="-45" // Offset the X position of the window
        //offsetX="15" // Offset the Y position of the window
        closeBoxMargin="5px" // Sets the box margin (could also be done in stylesheet)
        class="ui-infobox-dark" // Uses the dark theme made available by Google Maps for ExpressionEngine v3
        closeBoxURL="/themes/third_party/gmap/css/images/white-close-button.png" //Uses an alternative close button
        style="width: '400px'"
    }
        <h3>{title}</h3>
        <p class="google-map-contact-details">{m_address}{street}<br>{city}, {region} {postal_code}{/m_address}</p>
        <p class="google-map-contact-details">
            {if m_phone1}Phone: {m_phone1}<br>{/if}
            {if m_phone2}Phone 2: {m_phone2}<br>{/if}
            {if m_fax}Fax: {m_fax}<br>{/if}
            {if cf_firm_website}<a href="{cf_firm_website}" target="_blank">{cf_firm_website}</a>{/if}
        </p>
        <p class="google-map-contact-details"><a href="/directory/firm/{url_title}">View Full Profile</a></p>
    {/exp:gmap:marker}
{/cf_firm_location}
{/exp:channel:entries}

Any thoughts to why that might be happening? I've confirmed in the source code that the same firms are showing up twice. Thanks in advance!