As I understand, I got a bug.
When I add more than one watcher to an issue via Command Window the last user in watcher list becomes an assignee of an issue. And previous assignee changed.
1. Issue`s assignee is Assignee1
2. I write in command window "add watcher Assignee2 Assignee3"
3. Issue`s assignee becomes Assignee3 and watcher is Assignee2, Asignee1 is absent