曾经 has already been well explained here, but I sometimes see
as opposed to
Is there a difference in meaning or usage?
There are many verbs and adverbs in Chinese that hold the exact meaning without the second character. Chinese words have a more complete sound to them when they are in pairs of two characters, but it's not necessary to use the verb or adverb in its complete form.
Having the two different choices though gives the speaker more control over making the sentence sound more fluid, generally by pairing single character words with the single character form, and using the full verb with two character form.