If someone is using angular with ui-bootstrap module along with normal bootstrap HTML dropdown definition, there are also two clicks needed.
<li class="dropdown">
<a href="#" class="dropdown-toggle" data-toggle="dropdown">Dropdown</a>
[...]
</li>
=> Removing the data-toggle="dropdown" will fix the issue.
Opening the dropdown with one click will work after this.
Reference:
https://github.com/angular-ui/bootstrap/issues/2294