I am using preference manager for my Inovelli devices, and I see two of them for #51.
Preference '51. Disable Physical On/Off Delay' (name:parameter51)
and
Preference '51. Disable Physical On/Off Delay The 700ms delay that occurs after pressing the physical button to turn the switch on/off is removed. Consequently this also removes the following scenes: held, released, 2x, 3x, 4x, 5x tap. 1x tap and config button scenes still work. (firmware 1.19+)' (name:parameter51)
What's the difference?
As far as I know, those are the same. Then you turn off the delay, the switch will be much more fast to response, but this will disable the ability to use the multi-press functionality.
I believe the above is correct, and I don't recall this parameter changing at all since it was introduced (though they did add the related parameter 52 in a later update), so my only question is why Preference Manager is showing it twice. Perhaps you have two different copies of the driver installed, and the text is worded differently in each? See "Drivers Code" on the left-side menu if you want to check. This would also assume that you have instances of the device using both copies (harder to check if the drivers have the same name, though trying to delete the driver code would warn you it's in use).
This shouldn't be a problem in and of itself, though it may be confusing when you update driver code. Also, might not really be the case...just a guess. 
(EDIT: The below guess that one could be the LZW31-SN and the other an LZW30-SN is an even better guess...)
IIRC I have noticed that too. Would you happen to have a dimmer and an on/off switch?
@bertabcd1234 + @stevenkandres - your messages clued me in to look. Yes, they are the same parameter/preference - the strings in preference manager look different - one is for the switches and other is for the dimmer.
I've noticed that with a number of other parameters as well. It would be nice if they could all be done at the same time. @Eric_Inovelli
Yes, +1. Parameter standardization would be great to have!
What's the question here? Sorry, long day lol.
In the Preference Manager on Hubitat parameters for Red Series/Black Series/Switch/Dimmer show up differently. In the pictures below you can see there are two Parameter 51s, two different minimum levels, etc. It would be nice to have Parameter 51 show the same on the switches and dimmers so you can adjust them all from one spot.
I don't think that having the same text in the LZW30-SN and LZW31-SN will make a difference will it? I would doubt that Hubitat would merge two parameters in preferences manager based on the text description. It probably displays each preference for each device. LZW30, LZW30-SN, LZW31, LZW31-SN, etc.
I could be wrong though. Let me know and I will try to fix it.
@ericm - you are right, just making the strings consistent will not address this. I looked in the preference manager and there are many instances where the number and string are the same, but, they show as duplicates in the preference manager. It may have to be a rethinking of the preference manager design and layout itself if " Preferences by name" is going to be productive to use.
Who owns preference manager? Does anyone know where the code lives? Is it public?
Preference Manager is a built-in app, so HE Staff, and not public.
Download the Hubitat app
