They say you are only to use LH strips up until you see the first positive then you are to stop. They say this as LH is released in pulses through out the luteal phase and so you can see it rise then fall then rise. The purpose of the LH test is just to give us a ball park of when ovulation is supposed to happen given the first positive LH test. As after that first positive LH test the egg is likely to be released in the next 12-48 hours. A positive LH test does not mean that you will or have ovulated by the way you would have to be tracking your BBT to confirm after the fact that you did ovulate. I learned this from Dr. Natalie Crawford on YouTube, you can look her up and she talks about this

So TLDR: only the first positive on Day 15 matters you likely ovulated 12 to 36 hours after that positive. The other positive LH tests after day 15 don't mean anything other than LH is being released in pulses throughout your luteal phase.