Somehow I don't understand how you say Syncthing's disadvantage in comparison is that it's a P2P system[1], but at the same time your website says duple doesn't need a server.
Maybe I've misunderstood, but this sounds just like Syncthing with an always-on client - except for the file versioning, that sounds like an interesting feature to me.
I've been using SyncThing for over 4 years and while it has a few rough edges (synced/shared/global file ignore would be great) still it's been fully reliable and a generally great user experience. So if you're trying to cater to existing SyncThing users don't mince words to make it appear that something which you claim is a negative with SyncThing doesn't exist in your product - which it clearly does.
SyncThing has a huge advantage: years of trust. They have been around since 2013 and continue to crank out features and builds consistently. Duple hasn't been around for one year at the time of this writing. Beyond that it's clear from the Duple site that it's main goal is to take my data and file replication hostage via licensing fees. I'm curious how or why I'd donate before I've even installed the Beta (based on your click flow to even reach the downloads page)? No thanks.
Also, let's clarify something Duple has wrong...
Duple states: "Syncthing is P2P, so you get the disadvantages along with it e.g. all your devices need to be turned on at the same time. If not, you get a desynchronisation between your devices and create conflict." - This is wrong. You do not need all your devices on at the same time with SyncThing. Yes, it is true that it's good to have a device with a consistent state, however it's not required. The second part of the statement is FUD. When conflicts happen it's generally around odd permissions or file updates with regard to versioning. This was more problematic in versions prior to 1.0. At this point in time I haven't run into this issue other than because of disparate problems caused by file permissions which SyncThing does a great job preserving.
Duple also states SyncThing has no IOS support and yet, itself, has neither IOS or Android. Or Windows... Or an open source repo of what I'm supposedly using.
In my mind Duple doesn't compete with SyncThing and, really, never will. But here's the thing... Don't pretend to compete where you don't. SyncThing users aren't looking for Duple. You'd do yourself a better service to take that verbiage out because all it did for me was give me the impression that Duple is lying about competitors that they simply didn't take the time to understand. That leaves a bad impression in my mind.
I know this is a late reply,but maybe you'll see it.
Regarding the synced/shared/global file ignore, there's an imperfect but usable work-around:
You can use #include statements in your .stignore, so you can include another file that contains the global ignore list, then sync that file. You have to set up the .stignore with the include statement for all of your devices/folders, but after that, it's essentially a global ignore list.
I'm sure what they mean by all devices needing to be on and connected, is that if only one device is connected, there will be nothing to sync to. So if I take some photos while on vacation, but my desktop at home is asleep, then I can't sync my photos to it. So if I lose my phone before then, I'm sunk.
Of course this is fixed by simply having the desktop on all the time, which would then make it similar to a client server setup.
Well, they don't say that so it doesn't really matter what they mean. And they imply that by not having a device on all the time it is the cause for synchronization issues which is completely incorrect.
If I take photos on vacation and throw them in a sync'd folder the next time both devices are online they will resolve the new file delta between that shared folder. That doesn't imply I always need one or the other device online. The more devices syncing the less likely it is that only one would be online at a time, but again there's no requirement there for an always on device.
Anyway... SyncThing is fantastic for users who are willing to invest some time learning how the software works. Every paid for product seems to cater to the "it just works" mentality thereby sacrificing control to me, the user, to handle situations that can't be handled by overly simplified, cloud-first, lock the user into our licensing model solutions. And don't get me wrong, those are fine for many people. For users who want more control via more responsibility - then SyncThing is great. But I don't like how they are spreading FUD about it just to get some name association.
Syncthing does file versioning, although a bit noobishly. It just keeps n copies of the old file in a hidden folder with DATE_MODIFED appended to each copy.
Maybe I've misunderstood, but this sounds just like Syncthing with an always-on client - except for the file versioning, that sounds like an interesting feature to me.
[1] https://blog.duple.io/what-is-the-point-of-duple/