I wanted to help find some bug.
Ok, if there is a bug then I misunderstood you.
My impression was that you had copied the project files in a way they are not meant to be copied, resulting wrong references somewhere.
Copying source code between projects in editor is rather safe, as well as copying a Pascal unit file. Copying the project files is different and not recommended.
Anyway, if there is bug then we need clear steps to reproduce it.
One more thing about Lazarus projects and packages (this whole thread should be in Lazarus section):
Project's unit search path (-FU) should not point to external shared code.
For shared code you should always create a Lazarus package!
This is different from Delphi. There packages have their paths added to a global search path, used by all projects. It pollutes the namespace also for projects that don't use a certain package.
In Lazarus a project gets a dependency for a package. The package is found even if it is not installed in IDE because its location is remembered after it is opened once in the IDE.