If you do decide to consolidate your sites so that the same exact content would be served for both domains, then yes, this would be an example of duplicate content. While I wouldn’t call it deceptive, you would still fit the definition by serving the same content on multiple URLs.
Since Google may then see these two different domains as different sites, your consolidated content may not be ranked properly. For example, let’s say you have two links to your content, but each link goes to a different domain. Instead of two links to your content, Google may see it as one link to the UK spelling, and one link to the US spelling, splitting your ranking.
But, there is an easy way to handle this! First, choose your preferred spelling for the domain, and host your content there. Then, use a 301 redirect to direct any visits to the alternate domain to the preferred domain. Google can follow these redirects too, so a link to either spelling can be properly considered when ranking your content. Full Discussion Here
