What kind of word is "kembali" in this sentence? Is it like part of the verb? (re-)
Yes, (actually it means again)
So you mean "kembali" and "menduduki" belong together? I can't think of an example but can a word go between those 2?
Kembali can be used as a verb on itself too (meaning 'to return'), but here it is used to modify 'menduduki'.