Groups and different items-files

i want to split my big item file to many small ones. I want to get better visibility.

Do i have to define groups in every items-file?

I have one group “living_room”. I want to use it in the lights-item file and also in the rollershutter-items-file. Do i have to define it twice or is it enough to define it only in one of the items-files?

You should only have to define it once and will get an error if you define it twice.