I'm not sure what kind of doser you have, but when I was using autodosers to dose PPS Pro solution in my planted tanks, I had the same thing going on. I never put a check valve on because it worked in my favor to get a smaller amount of solution out with my highly calibrated, super precise 1 minute invterval timer haha... If this is something you're wanting to avoid, a check valve should do the trick.
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