Opeaning in new tab on click of repeater button in update panel

String url = "window.open('ProjectCompleteDetails?ProjectId=" + projectid + "','_newtab');";

ScriptManager.RegisterStartupScript(this, this.GetType(), "OpenWindow", "window.open('url','_newtab');", true);

or

ScriptManager.RegisterStartupScript(UpdatePanel2, UpdatePanel2.GetType(), "OpenWindow", window.open('url','_newtab');", true);

Will open in new tab but same tab again and again and not in other new tab ?????? Why