It sounds like you're running into:
https://github.com/JuliaLang/julia/issues/12508IJulia is already loaded, but probably in your Julia session you've done Pkg.update() or something which updated either IJulia itself or one of its dependencies. This causes IJulia to be recompiled when you try to load it again, but this fails because it has already been loaded. The workaround is just to restart Julia (just choose "Restart Kernel" from the IJulia/Jupyter menu).
