An operation has been defined in more than one place. Operations can be defined as a body or on a sequence diagram.
Consequences
It may cause confusion for developers when operations have been defined by a body field and on an object sequence diagram, as the body and OSD often will not match.
If they appear on more than one OSD this can make it more difficult to maintain the model correctly as both instances will need to be updated if any changes are made.
Should one instance of the operation OSD be updated without updating all instances, this causes confusion for developers as many operation definitions have been unknowingly created.
Strategy
You can merge the definitions or remove one of them. If you believe that multiple definitions are allowable in order to help developers realize the full responsibility of an operation then you can ignore the review item.