There currently is a unique constraint for the field SHA-1. The entries for installers by SHA-256 are added individually with the SHA-1 field empty when the policy is imported.
The database has a unique constraint for this field on SQL, which is why the error occurs.

When you add an installer to a rule, it is added one entry for each hash. If it is an SHA-256 installer and you add it to a rule, an entry for the SHA-256 is created. If the installer is both SHA-1 and SHA-256, two entries are created, one for each.