I ran across one hobbyist who has reviewed the same provider eight times, six of her alone plus two more reviews with another woman.

On this site, I'd guess something around 15 times? Just a blind guess.

 
I think a more interesting question is whether such a person's reviews past the first one contained any new information relevant for people reading his reviews, or was it all just to pump up the provider:s stock?

I have repeat offenders, but that seems extreme & I always wondered why they don’t cancel out if it by the same reviewer.

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There is a rule that you have to wait 90 days (or something like that) before you can submit another review of a provider you'd already reviewed.  I once had one of my reviews denied because I'd tried submitting a second review too soon.  
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One a long time regular whom I have seen over 10+ times in span of 4 yrs.. She asked me to review her after she moved to a different identity. I did..  

 
Another one asked me to review her again to help her get fresh reviews as she was coming back from retirement.. I did..

 
In both cases the new reviews were submitted right after the latest appointment.. So don't report me to TER.. LOL