Annan can be another spelling of the Arabic name Anan. You can read the details of this name here: Anan
Annan is also an Arabic word that means "slow actor", "sluggish". This is not used as a name due to its negative meaning. The difference between this word and the name Anan is that the name is pronounced with emphasis on the second syllable (as in Anaan), while the the word annan ("sluggish") is pronounced with stress on the -nn- part.
If you like the spelling Annan, you can use it as an alternate spelling for Anan, this is perfectly acceptable.