gtk-play: handle duplicate uri in playlist
gtk-play: handle duplicate uri in playlist

current logic does not play all the files from playlist if the list contains
a duplicate uris.

shakin/gst-player,amorenoc/gst-player,jabiers/gst-player,sdroege/gst-player,jabiers/gst-player,shakin/gst-player,JimmyOhn/gst-player,guiseco/gst-player,codomania/gst-player,guiseco/gst-player,sdroege/gst-player,jabiers/gst-player,sdroege/gst-player,sdroege/gst-player,codomania/gst-player,codomania/gst-player,amorenoc/gst-player,amorenoc/gst-player,codomania/gst-player,shakin/gst-player,guiseco/gst-player,JimmyOhn/gst-player,justinjoy/gst-player,JimmyOhn/gst-player,JimmyOhn/gst-player,codomania/gst-player,JimmyOhn/gst-player,sdroege/gst-player,justinjoy/gst-player,amorenoc/gst-player,justinjoy/gst-player,amorenoc/gst-player