I don't often come across the form but it makes sense and it has a bit different nuance from the standard form.
For example, when 知っていればいるほど is compared with 知れば知るほど in your example, 知っていればいるほど is more natural than 知れば知るほど. 知っている means 'you have already known about that and have the knowledge of that", so your example can be translated as "The more you have the knowledge the more you receive recognition as a witch."
On the other hand. 知れば知るほど doesn't have the nuance. It has a nuance like "even if you haven't known about that right now, but if you know that". For example. 日本語は、知れば知るほど難しく感じる. I feel that 日本語は、知っていれば知っているほど難しく感じる is a bit unnatural.
In addition, I noticed that I misunderstood your question from reading your comment. I answered the difference between 知っていれば知っているほど(ている form) and 知れば知るほど(standard form). As for the suru verbs and ている form, you can omit the stem of the second verb and the meaning doesn't change.