# K1576–K1580: duplicate FD and shared open description

Two descriptor slots may refer to one read-only open description. `dup` publishes a distinct FD only after reserving a free slot and a reference count. Sequential reads through either FD advance the same description offset by the actual byte count. Closing one FD leaves the other and its shared offset valid.

Beginning a read takes an in-flight lease. If the last FD closes while that read is active, the description stays live until the lease completes. Completion advances the shared offset, releases the lease, and retires the object only when both FD and in-flight counts reach zero. A repeated completion or operation on a closed FD is refused without state change. Overflow and full-table refusal likewise publish no partial owner.

This bounded two-FD model establishes ownership and offset behavior. A userspace `dup` and read ABI, disk-backed data, and product release acceptance require separate evidence.