"Have been seeing" is the present perfect progressive tense and active voice. So for example "Rick and Jane have been seeing each other for two years." That means they have been dating each other. "Have been seen" is present perfect (simple) but passive voice. Therefore, the doer of the action is unknown or in the "by" phrase. "The patient has already been seen by the doctor." In other words, the doctor has seen the patient. Hope that helps.

"I have been seeing the ducks flying over the river, all day. Some of them looked at me, therefore, i have been seen." This example is a bit unreal, but i think it's correct.