Is it common for people with bipolar disorder to be narcissistic too?

Bipolar/Narcissism. The two share some features, especially grandiosity & inflated esteem, and they may coexist in same person.
Similar, not same. In a manic episode there is an inflated sense of self worth and sometimes grandiosity, but during a depressive episode there is a decreased sense of self worth. Narcissism is a chronic and consistent air of superiority and a need for admiration which is actually due to a very fragile self esteem and increased sensitivity to criticism.