In my limited experience (ds1 age 9 was dx with Aspergers in January this year) I would say its fairly common, although it presents in different ways with different children.
I'm not sure how much help I can be though, because I feel a bit like we must have handled it monumentally badly with ds1, as he's now completely over-reliant on one friend and likely to have his heart broken when they leave primary school because of it. 
My ds has one best friend and he literally worships the ground this boy walks on. The boy concerned is, fortunately, an absolute gem and has been ds's communication and play facilitator, protector, confidante and all round best mate ever since they were in year 1 (they are now in year 5).
In the first year or so, there was a bit of a tug of love situation as his friend has one other friend in the class who he'd known for longer and who obviously wanted him as her best friend. This girl is particularly spoiled and used to getting her own way and also had a lot of fire power, if you will, as she could always invite the boy round to hers, where there was always the latest toy etc. Ds1 really struggled with this for a long time, but in the end his best friend saw through the girl's manipulation and took objection to her bad mouthing ds1 and decided he didn't want to be her friend anymore.
In some ways that was great, but in others less so, as ds1 is totally dependent on his friend at school, both in the classroom and the playground. His friend tells him what the teacher has said and what he needs to do in class, includes him in games and insists others let ds play or he won't play himself (he's very popular in the school so that holds a lot of sway with their peers). This situation isn't fair on the friend and both us and the teachers have tried to encourage ds to work and play with other children, although it has to be said - with little success so far.
Ds1 is obviously getting a bit older now and has recently become aware of the potential for him losing his best friend. His friend is allowed out to play on his own and is starting to mix with other children from their year who are also allowed out. In addition he has invited ds to a film night (ds is film/movie phobic) and also a sleepover party - both of which terrified ds and let to a lot of anxiety, little sleep etc, but which he forced himself to attend (well he did the film night - the party is this week ) as he's scared of letting his friend down, disappointing him and ultimately potentially losing his friendship as a result.
On top of that, his friends parents have told us he is going to go to Grammar School rather than any of the local secondaries and this sadly isn't an option for ds. Its highly unlikely that they will manage to keep up the friendship beyond primary and not only is ds1 going to be bereft, I also have no clue how he will survive secondary without his best friend alongside him. 
Prior to developing a genuinely brilliant friendship with this boy, ds used to fixate on certain children, absolutely convinced they were his best friend, but who usually barely knew he existed and certainly had no interest in being friends with him.
We have tried to guide his friendship with this boy, eg we have had him to play at our house a couple of times, but literally only a couple in the past 4 1/2 years so that we aren't encouraging the obsession. We've made a point of trying to encourage friendships with other children, but not very successfully as he is so rigid in his thinking that, whilst he will mix with other children, really he won't even entertain the idea of anyone else being 'his friend' - why would he, when obviously "X is my friend, so I don't need to be friends with Y".
This term the teacher has made a rule that he isn't allowed to work with his best friend in class, except for one particular subject, in an attempt to get him working harder on listening for himself and being more independent with his work. They're also doing some extra SEAL work around teamwork and co-operation, where all the children are having to switch partners for exercises.
We were hopeful that he had found a second friend recently, as another boy in his class developed a strong interest in one of ds's obsessive topics and they started to spend more time together - but then his mum told me they are moving to the other end of the country in a couple of months, so that's obviously not going to be a good relationship to foster.
Its a minefield isn't it, and so hard to know what to do when you aren't actually there with them to observe the friendship and help guide them.