I simplified a schematic and removed a lot of components, however they still appear in the list of devices to place on the board, and also in the list of errors for unconnected component pins.
It seem the project still thinks they are there even thoughi delted them from the schematic. They were never placed on the board.
How can I make it forget the removed components?
I originally created this schem by copying a schematic from an existing project schem into a blank schem in a new project.
I just tried this again - copying like that causes all sort of problemsā¦ some part numbers are reset, and some connections lost.
we really need a better way to fork an entire project. I thought copying the schem into a new blank project would be safer that cloning the folder in the fidner but it seems not.
Could you describe the exact steps how to reproduce this issue? Or create a video how you can reproduce it? And which LibrePCB version are you using?
If these components are really not used anymore, you could remove them manually from the file circuit/circuit.lp in the project directory. Just search for the corresponding component names and remove the corresponding (component ... ) blocks. But make a backup first 
I havenāt found a simple way to reproduce it yet. I ended up redrawing the schem over from scratch and the problem hasnāt happened again.
OK I see. If you ever find out how to reproduce it, please let me know. This problem was already reported some time ago, but I was never able to reproduce it so I was not sure if it still existsā¦
Iām seeing this repeatedly.
Try this procedure to replicate -
make a scehmatic including devices that have more than one symbol required to represent them in the schem (like a dual opamp)
copy that schematic from another project into a new empty project.
remove all the symbols for the dual opamp.
replace it with a new similar device requring dual symbols (this bit may not be needed)
look in the device placement list in the baord editor and note the removed device is still there even though itcanāt be seen on the schem.
actually every component shows in the ERC as unconnected. I think copying the schem to a new project is basically broken. I giuess thats the same as I found earlier.
Aaaah, thanks for the hint! 
Found the issue and fixed it so this problem should be solved with the next release:
Except that it is annoying, this bug is not critical for the integrity of a project. The workaround by manually removing these components from circuit.lp will safely clean up the stale components, so this is the way to go until the next release is available.
great!
Iām really looking forward to the next release - it sounds like there are lots of things fixed already.