Comments: ** Comment from web user: strongwind **
I can confirm its occurring. Although, it just seems random. It happens with all our developers.
Sometimes, after creating a new Resource, instead of adding to the existent file, it creates another "ResourceName1.Designer.cs", identical the first one, but adding the new created resource. It also adds this file to the project. The solution for this, is to compare the created file with the old file, add the new created resources manually to new one, delete and remove the "1.Designer" file from the project.
My guess is some permission to write on the file. Windows 10 seems to complain a lot about righting to existent files, and after 1 or 2 more tries, it's succeeded. I've seem this behavior with other programs. It generally fails and asks to try again. Perhaps, when ResX Manager fails to write the file, it just creates a copy with a counter at the end, instead of failing and asking to try again?