scan.kt

/**    
 * Emits repeated values only.
 */
fun <T> repeats(): Observable.Transformer<T, T> =
            Observable.Transformer<T, T> { observable ->
                observable.scan(Pair(null, null), {previous: Pair<T?, T?>, current: T -> Pair(previous!!.second, current)})
                        .filter { x -> x.first == x.second } // only let repeated values go through
                        .map { x -> x.first } // take the repeated value from the Pair and drop the Boolean
            }

/**    
 * Example usage for tab bar selections.
 */
Observable.just("Home", "Browse", "Home", "Home", "Profile", "Profile", "Home", "Browse", "Browse", "Profile")
                .compose(repeats())
                .subscribe { selection -> toast("Clicked on $selection twice!") }