Categories "duplicated"

While adding a new Edocman Category "Migranten 1580-1940" I noticed that some Categories were "spontaneously" duplicated (see attachment ID's 6 and 7 ). The Category "Midwinterpraot" originally had ID 8 and was also duplicated. The "Migranten 1580-1940" addition remained single. The original Documents were indexed, the duplicated ones not.
As ID 8 only had 8 Documents I decided in the test environment to delete the non indexed documents and to delete the Category. To my surprise the deleted Category popped up soon thereafter as ID 18 (see attachnent  
Next I deleted the origial and the duplicated Categories in the live website and created a new Category "Midwinterpraot" ID 13, which is no longer duplicated!
As the other Categorie have large numbers of Documents, I find it too risky to delete any duplication without a full understanding. Do you have a suggestion?