PHP Code simplest example

function SplitOnFirstUnique($s){
    $len = strlen($s); 
    $output = [];
    $newstring = '';
    for ($i=0; $i < $len ; $i++) { 
        $newstring = $newstring.$s[$i];
        if(!in_array($s[$i] , $output  )){
            $output[] = $newstring;
            $newstring = '';
        }
    }
    return $output;
}

PHP, 317 bytes

function SplitOnFirstUnique($s){
    $len = strlen($s); 
    $output = [];
    $newstring = '';
    for ($i=0; $i < $len ; $i++) { 
        $newstring = $newstring.$s[$i];
        if(!in_array($s[$i] , $output  )){
            $output[] = $newstring;
            $newstring = '';
        }
    }
    return $output;
}

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