PHP Remove duplicates from multidimensional array based on key

I have a multidimensional array ($array) in which the entries look like:

{ ["upload/example.gif"]=> array(5) { 
     ["title"]=> string(12) "This is me" 
     ["excerpt"]=> string(24) "This is a photo of a tree" 
     ["img"]=> string(42) "upload/example.gif"
     ["link"]=> string(23) "http://www.google.co.uk" 
     ["source"]=> string(6) "custom" 
   }
}

I need to be able to remove any duplicate values in $array based on the key. So if my array was:

$array = array( ["upload/example.gif"] => etc....
                ["upload/tree.gif"] => etc....
                ["upload/example.gif"] => etc....)

I would be able to remove one of the ["upload/example.gif"] => etc.... arrays.

I have tried:

$array = array_map('unserialize', array_unique(array_map('serialize', $array)));

but that didn't work.

Thanks in advance.