Yes, that's fine. There's no reason at all why you can't say that. The first 'for' belongs to the phrasal verb 'been looking for', while the second is a preposition of time belonging to the phrase 'for over a decade'. In spoken English, they wouldn't even sound like the same word. The first 'for' would be in its full form, while the second would have its vowel reduced to a weak schwa sound.

Yes, that's totally ok! :)

It's fine. I disagree a little with Laurence in that I don't find it clumsy. It is quite common.

You do hear people say stuff like this (probably guilty myself), but in reality it's clumsy and probably best to avoid in writing.