Populating drop down dynamically

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Hello,
I have a drop-down search field that is being populated using API and xml. This works fine, apart from the fact that I can't seem to get rid of repeated elements. For example, the code below retrieves the 'activity' of a tour. But more than one tour is, lets say, 'kayaking' so I end up with a drop-down that has 'kayaking' listed multiple times.
I only need to list this activity once.

I've tried using 'unique_array' as well as many other things with no luck.

Any ideas very much appreciated

<select name="Activity" value=""> 
               <option disabled selected> -- Select activity -- </option>
               <?php 
               foreach ($search->tour as $tour){ 
               $activities = $tour->xpath('custom1'); 
               foreach (array_unique($activities) as $activity) {
               ?> 
               <option value="<?php echo $activity; ?>"><?php echo $activity; ?></option> <?php }}?> 
               </select>