ZSE Controller voiced concern regarding the apparent dual control of Data Blocks available during certain conditions utilizing the ERAM equipment.

Narrative

P80 Approach had the MMV airspace (SFC-050) which belongs to ZSE but is given to P80 at times for MMV traffic. It encompasses the MMV airport and butts up against P80 airspace. There are no combine sectors messages with the release of this airspace; just a HOT airspace delegation in the EDST with an associated altitude which designates that area. P80 cleared the aircraft off MMV and departed it. Since the airport is in my airspace even though it was delegated to P80 at the time; the Data Block on Aircraft X auto acquired on my scope showing I had control of the Data Block while flashing hand off HS06 at me. The aircraft then crossed the normal designated P80 airspace boundary line into P80 airspace while on the Flight Plan Route when P80 called me and mentioned about the 'C' tag and that they were unable to make an automated hand off. At that point we did a manual hand off. The aircraft reversed course and reentered my airspace still flashing HS06 even though I still had control of the Data Block. When the aircraft crossed the P80/ZSE boundary into my sector the Data Block hand off was automatically accepted by ERAM and stopped flashing. On this departure; ERAM gave both ZSE and P80 dual control of the Data Block which caused problems with the associated auto-hand off function. Only one sector/facility should be able to have control of the Data Block at any one time. Recommendation; prevent two facilities from having dual control of the same Data Block in order to prevent this occurrence and prevent the hand off from being automatically accepted.