Happens every time - repo is just to make two different parts and a WeldConstraint, then assign the Part properties and press Ctrl+D.
Not sure if this is intended behavior, but it’s really annoying either way - Ctrl+D is basically my goto for creating copies of objects quickly.
If this is intended behaviour, a warning should just get printed to the output that one of the constraints will be disabled and won’t work.
This is a known issue and will probably be fixed sometime within the next couple months.
After further investigation, this is actually not a bug and is working as intended.
There can only be one WeldConstraint between the same two parts. Adding another one will delete the old one. You can notice this by inserting a new WeldConstraint in the Workspace and setting Part0 and Part1 to two parts that are welded with a WeldConstraint. The old WeldConstraint will be destroyed. This is intentional.
This is what happens when you press Ctrl+D. It’s cloning and then destroying the existing weld so it appears to not work… And it’s not, but in a way that’s expected.
Should we maybe make this more clear? E.g. instead of duplicating with Part0/Part1 intact, leave Part0/1 blank or only set Part0. This way developers can tell what’s happening, and they’re more than likely going to change either Part0/Part1 anyway since there’s no reason to have two WeldConstraints with the same Part0/Part1 combination.
Mm, I agree
I already wrote a sentence covering this scenario 
Duplicate should always work, no matter what – that’s one of the reasons why I use it – and if it working will stuff something up, that’s what warnings and small property changes are for.
Duplicating objects I get. Duplicating a part with a WeldConstraint in it will work and duplicate the weld with the part, and point it to the new part. That makes sense.
I don’t understand why you’d want to specifically duplicate an existing WeldConstraint in place? Honestly curious what your workflow use case looks like here.
If you want a new raw WeldConstraint with parts not set, there’s Advanced Objects (which does have a bug where it inserts WeldConstraints into workspace. That, I’m fixing).
If you want to weld that part to more parts you can do multi-select and hit the weld button which will weld all touching parts to each other, or click weld and click each part to create a new weld. You can add a hotkey for this action to make it quicker as well.
I’m not much of a builder, so I probably don’t represent the building community on this specific issue.
When I am welding, my habit is to manually make a new weld, assign it’s properties, then duplicate it so I may only need to change 1 property to get the new weld working.
My workflow pretty much stays the same for everything else I do - UI design, organising, etc.
It’s minimalistic and is supposed to work for everything.
Assigning hotkeys doesn’t really fix the issue (imo, considering I’d need to adjust my workflow specifically for welds) but it’ll do I guess.
A warning or something at least, though, would be appreciated for people who don’t know what’s going on.
I’m mainly arguing this for consistency’s sake - basically everything else works with duplicate that I know of, regardless of properties, so it is just really weird that welds don’t work.