Indicates that something happens with some regularity, but not always. Often used in formal or written contexts. Can imply 'more often than not' or 'it is not uncommon that...'.
He sometimes forgets his promises.
Problems like this are often overlooked.
Kana spelling; used when the kanji is avoided or for clarity.
Alternate kanji spelling; less common but still encountered.
From the reduplication of the kanji 往 (going, past), literally 'going and going', which evolved to mean 'sometimes' or 'often' in the sense of recurring events.