It is never used else

I am doing some work with grails and my problem is that I can't never do the program go through the else part. My code is the following

while (pois.hasNext()){
        def poi=pois.next()
        def site = Site.get(poi.site)
        if (!sites.contains(site.id)){
            sites.add([id:site.id.toString(),name:site.name])
        }else{
            println("It has been previously added")
        }
    }

It should go through as I have checked my DB