Checkbox issue: Checked="checked" not working

I am having a weird problem with checkboxes; in this piece of code:

<div class="two-column-layout">
    <input type="checkbox" name="externalSystemEndPoint" class="hidden" />
    <input type="checkbox" checked="checked" id="endPoint-{{endPointCode}}" name="externalSystemEndPoint" title="{{externalSystemName}}" value="{{endPointCode}}" {{disabledAttribute mandatory}}/>
    <label for="endPoint-{{endPointCode}}" class="checkbox-label">{{externalSystemName}}</label>
</div>

Even the second checkbox (with id="endPoint-{{endPointCode}}") it's appearing, the effect of the attribute checked="checked" is not being applied.

Any hints?

Thanks in advance.