Weird error in omnigraffle

I have multiple
“More than one graphic in the saved document has an ID=0. Changing the id of one of them.” when opening my omnigraffle file. What does that mean?

I get the same thing when I open some of my stencil files. I’ve copied my stencils to a new stencil file and saved it and that seems to have cured the error. No matter what I did to the original file it would never save the IDs to the older file and stop giving the ID=0 error.