what is the grammar form where 找 is used twice in the sentence 林先生找工作找了多长时间?
Though I don't believe it is necessary for the example sentence you gave, there are cases where you're required to repeat the verb. I'd just post this as a comment if I could but I think this image sums it up nicely:
https://resources.allsetlearning.com/chinese/grammar/degree_complement