If only i could use all my 50 votes here i would, this happens to me a lot, only just found this post trying to make a ticket to support, right now i'm trying out https://huere.net/trakt/duplicate_removal/ and see if that works.
Atm my most watched movie is a movie i watched back in june '13, 194 times the exact same time and day.

Not sure if this is a bug in the KODI addon or the Plex plugin.
sp10's tool is showing 1 scrobble and 10 watches for the exact same time with different ID's.
See below:

3x8 Chapter Eight: The Battle of Starcourt (10 play(s))
2019-07-15T11:19:15.000Z - ID=4649978466 - TYPE=scrobble
2019-07-15T11:19:15.000Z - ID=4661071679 - TYPE=watch
2019-07-15T11:19:15.000Z - ID=4663849539 - TYPE=watch
2019-07-15T11:19:15.000Z - ID=4663945439 - TYPE=watch
2019-07-15T11:19:15.000Z - ID=4664119930 - TYPE=watch
2019-07-15T11:19:15.000Z - ID=4664149196 - TYPE=watch
2019-07-15T11:19:15.000Z - ID=4664294375 - TYPE=watch
2019-07-15T11:19:15.000Z - ID=4664478104 - TYPE=watch
2019-07-15T11:19:15.000Z - ID=4667051444 - TYPE=watch
2019-07-15T11:19:15.000Z - ID=4667098063 - TYPE=watch

Maybe at an option to prevent an item to be added twice say, within the play time of that item

See https://huere.net/trakt/duplicate_removal/ for removal.

My config is mostly plays on infuse on Ipad, which track and mark on my kodi on PC. that point is working great.
the problem is every time kody freezes, it add another duplicate at the same timestamp... and at some point, the kodi crashes a lot...
I have just checked and for game of Thrones, I have 10 data entries at the same timestamp...
if we multiplied by 3 or 4 seasons for this show plus lots of other ones on my server/HDD, it's quite some big addition to our stats...

on the contrary, all the manual checked stays as single ones...

I often do a manual cleaning, but it takes some time (even with the help of the "most plays" sequencing), it would be great if trakt could do it by itself.

Actually just happened again. Probably going to turn on debug logging 24/7 again... Going to have to look at the script once I have the motivation but how likely is it that the response from trakt is not sending all data and that is why it sends them all again (or is the response code gospel)?

Do you mean the generation of the duplicates? When manually doing stuff I only get additional plays but never duplicates. What I am referring to is additional plays at the exact same timestamp generated from syncs from various media center plugins or mobile apps. I only occasionally try a mobile app so my main contender in my case would be the Kodi plugin (last time I cleaned up I had tripple plays for most :/).

So is this limited to manual play adds? I don’t use a hyper-manual approach like some so am curious.

I haven’t had any problems with duplicates, even when I expect them.

Or is this always/often happening when you check in at the same time as starting a new play???

I’m new-ish to Trakt (at least VIP/using my account actively), so I don’t think I’ve used the “Check In” function more than once or twice.

So if someone had an answer to especially my last question, it would be a big help. Thanks.

Scrobbles have a check to make sure the same data isn't inserted multiple times. I'll need to look into this for the manual watched history sync. It is not quite that simple since adding more lookups for every play will add a lot of performance overhead. This is likely a Trakt API v3 type of thing.