Cannot enable iCloud photo library on iMac so no new Photos in Photos app

iMac used to share and download the photos from all my devices. 10.10.3.

Photos app wasn't getting updated pictures, investigated and noticed iCloud photo library was disabled.

iCloud photo library used to be enabled, now it is not, I do not know why or when it disabled in the first place.

When I try and re-enable it treats all the photos on the iMac as new even though they are the same photos stored in iCloud Photo Library.

An obvious ploy by Apple to get me to buy more storage 😉

Is there any easy way to fix this? Or must I somehow get all the photos off the iMac and re-enable and then bring the photos back in? But I expect that would lead to duplicates.

Thanks,

Robert