This can happen when an item # is already taken that matches the ID of the new item being auto-generated. For example, if you have an item # that already exist in SR with a value of 100123 and you try to auto-generate a new item #, and the last auto-generated item # was 100122, Heartland Retail will automatically generate the new item with an item # of 100123-1.
To resolve this issue, the support team has a script to reset the item # sequence to a value higher than the highest item # which will avoid this from happening going forward. If you'd like to have that script run within your account, please reach out to HRetailsupport@heartland.us and reference this article!