Johanna is the female adaption of the male name Johannes, which, in English, is commonly known as "John", from the biblical figure. It used to be quite common in western europe to name children after either (grand)parents or biblical figures, or after (grand)parents who were named after biblical figures. I guess you get the drift. Female adaptions were quite common as well, and since most people didn't really know the old testament, the only females name they knew from the bible were Maria and Eva, which becomes problematic once you got more than 2 daughters. So, Johannes became Johanna, Petrus (Peter) became Petra, Paulus (Paul) became Paula. And there's a few dozen more like that.
These names are becoming less common though, too many parents start naming their children after footbal players ("Yari".. Ughh!) and teenage Disney sluts (soooo many Mileys suddenly).
