If I manually suppress one of those two layers, the original one also disappear.
Same if I manually delete their elements one by one.
If I deploy the layer "(2)" and suppress its elements, it's fine. When I delete the last one, the layer disappears along with the same name layer without the "(2)". This no matter in which order I suppress the elements.

I installed Webdesigner in a VM to see whether the PC has a problem, but it behaves the same way, with the same errors.
Also launched it and opened my document, made sure to have an empty clipboard, duplicated one of the pages and it crashes immediately.
I don't know where those duplicate layers come from actually...

Also, once it crashes, then I have a second error message: Spurious internal error has occured.

Steve