3D model
Duplicate placed parts in your design.
A part sits twice in exactly the same spot. In the 3D model you see one, on the BOM there is one too many. Order on that and you end up with a part too many in the workshop.
Published 21 September 2026
01 What it is
Duplicate placed parts, briefly explained.
A duplicate placed part is the same part that appears more than once in the design, with exactly the same position and orientation. The instances coincide, so you only see one part in the model.
It also applies to whole subassemblies. If a subassembly is there twice, everything below it counts double on the BOM: sometimes dozens of lines at once.
How you notice it
- The quantity on the BOM is higher than what you count on the drawing
- The weight of the design is higher than expected
- A position on the drawing points to two lines in the tree
- When selecting in the CAD you click through one part onto the other
In a check
This shows up as error in the check of the 3D model. You judge what has to be done about it.
02 Example
What it looks like.
Example design GRP-2210 (demo)
- Part
- FLG-050-PN16 flange
- In the model
- 6 instances, 2 of them in the same spot
- On the BOM
- 6 pieces
- What you need
- 5 pieces
The two coinciding flanges have exactly the same bounding box. On the drawing the position appears once, so the error only shows when you put the model and the BOM side by side.
How it comes about
- Copy and paste in the design, after which the original stays in place
- A pattern or mirror that also places the part on the existing spot
- Inserting a part again after changing the name or part number
- Two people working in the same design who both add the same part
03 Approach
Find, fix, prevent.
How to find it
In the CAD itself you almost never see it, because the instances cover each other. It is easy to establish, though, by comparing the positions of all parts with each other.
- 1 Open your STEP file in the free check on this site.
- 2 Under the findings, look at the lines about duplicate placements: per part it says how often and in which places.
- 3 Click a line to see the spot in the 3D model.
- 4 Compare the total quantity with the BOM: the difference is usually exactly the number of duplicate instances.
Fix
- Remove the surplus instance in the CAD, not in the STEP export.
- Then check whether patterns, mirrors and dependencies are still correct: sometimes something was attached to the instance you remove.
- Export the STEP and the BOM again, at the same moment, so you do not compare two versions.
- Go through the positions on the drawing if the quantities there were filled in by hand.
Prevent
- Use patterns instead of copy and paste, so the quantity stays in one parameter.
- Let the quantity on the drawing come from the BOM, not filled in by hand.
- Check the model for duplicate placements before release, not only when purchasing.
With BOMproof
Every release, without having to think about it.
BOMproof puts your STEP model, your PDF drawings and your Excel BOM side by side and points out every deviation: with a pin in the 3D model and a balloon on the drawing. You tick off what is correct.
04 Frequently asked questions
Duplicate placed parts.
01 Why can't I see the duplicate part in my CAD package?
Because the two instances lie exactly on top of each other. You see one shape, and the tree shows two lines that both look legitimate. Only when you compare the positions, or move one instance, does it become visible.
02 Does a duplicate subassembly also count as one error?
In the check it does: you get one line for the subassembly, with how many parts hang below it per instance. On the BOM it does carry through into all those underlying lines.
03 Is a part never allowed to sit twice in the same spot?
There are exceptions, for example a simplified representation that lies over the real part. That is why it is a finding and not a rejection: you judge whether it is right.
05 Often seen together
Other errors from the same corner.
You never want to hunt for this error by hand again.
Upload your model, your drawings and your BOM and let BOMproof check them against each other. Book a demo with your own design.