Annaliisa, I think you put your finger on the key question: is it interfering with other activities in a way that's a problem?
Our DS has Asperger's and has gone through period of repetitive writing and drawing that were definitely a problem: same picture over and over, or long strings of math problems of the same type. We typically would explain that he's "stuck" and redirect him to another activity.
I wouldn't have too much trouble with his having strong and well-defined interests (although that too is an Asperger's symptom that we manage); for me, the criterion for intervention is whether there's growth/development/learning happening through the special interest, or whether it just appears to be "stuck" and repetitive.
DeeDee
