I fired Xojo 2019r3 to test something and have the bad idea to load an existing project (of mine).
The I left it and fired 2015r1.
In the Open Recent dialog I saw a second occurence of the project I opened a minute ago. A click to choose it and I get a message that the file does not exists and ask if I want to delete it. I said Yeah !.
So last week current project wrote where it do not have to (or is it a feature ?).
Or I better stop downloading these funky IDEs versions 
did i understand that 2015r1 & 2019r3 share the same recent list?
I do not know where the recents are saved
I do skip lots and lots and new versions waiting for a stable one. So, when I try a new one, I use a copy of one project with all resources INSIDE. Never something I can risk, never something with resources outside THAT folder and subfolders. Never got things mixed.
Rick, your answer display a strong experience, and you are right.
Also (in the same veign), never change the IDE version while developing a project (worst: do not change version some times before delivering an application)
Let the discover time to when you are not in a hurry !
In the PLIST for the IDE on macOS, in the registry on Windows for the Xojo IDE
Yes its shared across versions
oh oh
Thank you Norman.
Markus: Oh Oh too 
I searched on my MacBook Pro, (system files and invisibles included), but I do not found it.
I would have been surprised (and annoyed) if the list of recent projects had stopped being shared across versions. When I install a new version of the IDE I expect to have a quick access to the projects I have been working on.
i think its app data and it need at least copied or display a xojo version number in recent list.
so it would not appear in older ide.
That is not what I wrote.
In loading a project with 2019r3, a new entry for that project was added to the list. Just if the avoid duplicates method does not worked anymore.
That is something relatively recent. It tooks me time to understand that.
I knew there was somthing wrong, but I do not undestand what.
Edit
Nota: this is different from a same name in different folders.
In fact, because of the above, I started to no more use that window and load the project using drag and drop (to be sure I load the last version and do not skip some work done previously or whatever).
[quote=472822:@Emile Schwarz]In loading a project with 2019r3, a new entry for that project was added to the list. Just if the avoid duplicates method does not worked anymore.
That is something relatively recent. It tooks me time to understand that.
I knew there was somthing wrong, but I do not undestand what.[/quote]
Maybe the current version of Xojo did save the project in a different format that a 5 years old version doesnt understand?
Thats been true for more than a decade
[quote=472822:@Emile Schwarz]That is not what I wrote.
In loading a project with 2019r3, a new entry for that project was added to the list. Just if the avoid duplicates method does not worked anymore.
[/quote]
How Xojo determines whether there should be a new entry or not relied on folderitems and since there were significant changes to folderitems for recent versions it probably gets that comparison wrong and things there should be a new entry
And however it saves the data now may cause old versions issues as well
Norman: you are probably right.
I wanted to create a brand new project minutes ago and watched the two similar projects.
After sometimes, I notices one path started with Macintosh HD: (my boot disk name) and the second started with /Volumes/Macintosh HD:.
[quote=472831:@Emile Schwarz]:
After sometimes, I notices one path started with Macintosh HD: (my boot disk name) and the second started with /Volumes/Macintosh HD:.[/quote]
Was the second saved in Catalina? OTOMH that might have caused the difference in the paths.
I would expect the difference is which version of Xojo you used
I believe a Norman expectation is a good one 
Catalina ? Vade retro Satanas !
El Capitan is talking here