I have a P2S with AMS2 Pro and every time I start a new print it completely purges the nozzle even though its using the same filament as the last print. Is there a setting that I am missing somewhere to prevent this?
There is not. The printer does not remember the filament used last and thus it purges at the start of every print.
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To add on what @JonRaymond said…
When you have an AMS connected, it unloads the filament at the end of the print, therefore it needs to be loaded again at the next print.
You are always going to purge a little bit because of the auto flow calibration.
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It will also home four or more times, and so forth 
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yeah I’ll be the first to admit that I don’t know the intricacies of all the preparation that it does at the beginning of a print and there is probably a good reason for it but certainly if you are printing always with the same filament one would think there would be room for optimization and efficiency.
What is the reason for retracting the filament after each print? Also why does it not save the last filament used information?
The reason for retracting the filament after the print is to prepare the AMS system for the next print. If you are running off of a external spool, it will not do this. The AMS system always assumes that you are going to be doing some sort of multicolor.
I think that they see last filament used as not so useful. It does not make a difference most of the time when the difference is only the AMS feeding it to the print head.
It is a good thought, but the way that I look at it, you are going to be purging the small amount that is in the nozzle anyways. Once a filament has already been melted, the quality declines. Better to get rid of it.
I don’t understand your point about retracting the filament in prep for the next print, if I am using the same filament what prep is needed? I understand that some amount of purging might be necessary to purge stale filament but it doesn’t have to purge it 3 times if using the same filament. Furthermore a lot of people use the AMS for running multiple reels of the same filament for when it runs out. I don’t print multi color prints because of the enormous waste. It seems like a no brainer to me to save the last filament type.
You do have the option to edit your start and end gcode to disable it.
Curious, is this a P2S thing? I thought I read the H series remembers the last filament and does “less” purging if it’s the same as last print but maybe that is just the H2C, I have a D.
Now to those wondering why it does it? I was printing a 5 plate multiple pieces model and I had canceled a plate due to an adhesion issue and went to bed which will leave the filament in the nozzles and let them cool down. This particular model had prime tower disabled and all the purge volumes set to 0 factor for efficiency I guess which I have never seen quite like that. The next morning when I started back up I had a whole mess of a blob on the support nozzle when it got to that layer since it didn’t do any prime or purging.
The H2C remembers what was used last in the right side nozzles, the nozzles have magical little com devices mounted inside them.
Are you sure what you’re seeing is all purge? There’s also dynamic flow calibration which may extrude twice before a print, if enabled. So that would be 3 times total including the initial purge/load.
[In my H2C experience the dynamic flow often runs even when the same nozzle and filament have just been used for a print… I usually don’t mind, but can always disable it if I think I know better than the printer.]
-Max
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I might be confused with purging and dynamic flow calibration, but forgive my ignorance, why would it need to recalibrate if using the same filament?
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I doubt anyone except BL could state the specific conditions under which auto calibration is performed (and I doubt they will). From my observations it errs on the side of caution, but perhaps that’s a generous way to look at it (also this may change in future firmware). It’s easy to disable if it bothers you, and it should remember your last choice next time you send a print.
-Max