Nobody says this sentence in English
He likes to swim often.
That'd be a different sentence in German: Er schwimmt gerne oft.
Thanks. However, the English translation Duo gives is strange sounding to a native English speaker.
This is a badly put together sentence in english
"oft und gern" sounds better with a different verb, i.e., Er kommt oft und gern," "he comes gladly and often [to visit me]. A little old fashioned, but possibly serviceable as a translation of an older novel.