My father, passing someone on the street on [שבת](http://judaism.stackexchange.com/tags/shabbat/info), says "גוט שבת, גוט שבת" (Yiddish, "good sabbath, good sabbath"). I thought it might be his own [idio-](http://en.wiktionary.org/wiki/idio-)custom — until I came across someone else who does the same. Neither could really explain to me why he does it, though the one not my father told me "that's the _minhag_ [=custom]!". Does anyone know where this comes from or why people do it?