这句没有错,这是个定语从句,LZ可以这样理i don't believe the people（ who i don't think ）could do it,括号里面是一个由who引导的定语来修辞the people.也就是说,i don't believe the people could do it 才是主句,（who） i don't think是个用句子充当的定语（who可省）.LZ可以看看定语从句相关的语法知识,就能理解这句话了.
其实改成这样要好理解些：i don't think the people who i don't believe could do it .