Protocol 9 does a straight circular mapping. Some people seem to feel that is correct. Protocol 39 does a mapping using a modified gray code that matches the old aculink reporting and my 01512 display. I believe some of the aculink parsers (ipwx, haculink, etc.) went through the same discovery process. I don't know if there are two versions of the 5-n-1, or if the people who feel the original circular mapping is correct just haven't noticed.
I'm pretty sure there's only the gray-code mapping. It comes from a plastic optical encoder disc inside the 5n1. It wouldn't make a lot of sense for them to change that for different production runs.
In any case, it's really easy to fool yourself that the "circular" mapping sorta works, but when you really look at it in a detailed, methodical fashion, the gray-code version is correct.
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Regarding suppressing duplicate messages: The Acurite devices send the message multiple times in each message burst. In a number of cases, the received signal strength indicator on acurite gear is not the actual signal level, or signal-to-noise ratio, but is an attempt indication of quality judging by the number of times the message was heard. Note: IIRC, Protocol 9 will suppress duplicates, but assumes that only a single 5-n-1 will ever be heard.
Hmm... on the displays they subtract a bar every time an expected data packet is missed, when it drops to zero then the display starts rescanning.
On the bridge/SmartHUB, there's an analog voltage on the RSSI pin of the radio module that represents the signal strength. I don't know the details of how they are computing that, but the bridge's microcontroller converts that into an integer up to 4.