I think I had the same problem, 2 things need to be checked:
1) NZBdefault folder must be different from nzb autoimport folder in the setup (to prevent neverending loop) (see other post about this issue).
2) If you work with an restricted account (another account than administrator), check the user rights of the files and both NZB folders. Otherwise Altbinz may not be able to delete the nzb file from autoimport(or move to the defaultnzb folder) and keeps starting the download again and again. Check properties of the files and folder, there should be a security tab I believe.
hope this helps.