HP ProCurve 6120G/XG Manual page 397

IPv4 Access Control Lists (ACLs)
General ACL Operating Notes
< acl-list-# >: Unable to apply access control list.
The indicated ACL cannot be applied to an interface because an ACL is already
assigned to the interface. The command fails for all included interfaces,
including any that do not already have an ACL assigned.
Duplicate access control entry.
The switch detects an attempt to create a duplicate ACE in the same ACL.
9-73